plugins: better handle exceptions in __init__

When importing a plugin, if it raised an exception in its `__init__` file, we
ignored it, and still loaded the plugin, in a potentially half-broken state.
This is because maybe_load_plugin_init_method only calls exec_module_from_spec
if the plugin is not already in sys.modules, but exec_module_from_spec
will put the plugin into sys.modules even if it errors.

Consider this patch to test with, enable the "labels" plugin:
```patch
diff --git a/electrum/plugins/labels/__init__.py b/electrum/plugins/labels/__init__.py
index b68127df8e..0d6d95abce 100644
--- a/electrum/plugins/labels/__init__.py
+++ b/electrum/plugins/labels/__init__.py
@@ -21,3 +21,5 @@ async def pull(self: 'Commands', plugin: 'LabelsPlugin' = None, wallet=None, for
     arg:bool:force:pull all labels
     """
     return await plugin.pull_thread(wallet, force=force)
+
+raise Exception("heyheyhey")

```

I would expect we don't load the labels plugin due to the error, but we do:
```
>>> plugins.get_plugin("labels")
<electrum.plugins.labels.qt.Plugin object at 0x7801df30fb50>
```

Log:
```
$ ./run_electrum -v --testnet -o
  0.75 | I | simple_config.SimpleConfig | electrum directory /home/user/.electrum/testnet
  0.75 | E | p/plugin.Plugins | cannot initialize plugin labels: Error pre-loading electrum.plugins.labels: Exception('heyheyhey')
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/plugin.py", line 148, in exec_module_from_spec
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/user/wspace/electrum/electrum/plugins/labels/__init__.py", line 25, in <module>
    raise Exception("heyheyhey")
Exception: heyheyhey

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/plugin.py", line 167, in load_plugins
    self.maybe_load_plugin_init_method(name)
  File "/home/user/wspace/electrum/electrum/plugin.py", line 293, in maybe_load_plugin_init_method
    module = self.exec_module_from_spec(init_spec, base_name)
  File "/home/user/wspace/electrum/electrum/plugin.py", line 150, in exec_module_from_spec
    raise Exception(f"Error pre-loading {path}: {repr(e)}") from e
Exception: Error pre-loading electrum.plugins.labels: Exception('heyheyhey')
  0.75 | D | util.profiler | Plugins.__init__ 0.0030 sec
  0.84 | I | simple_config.SimpleConfig | electrum directory /home/user/.electrum/testnet
  0.89 | I | __main__ | get_default_language: detected default as lang='en_UK'
  0.89 | I | i18n | setting language to 'en_UK'
  0.89 | I | logging | Electrum version: 4.5.8 - https://electrum.org - https://github.com/spesmilo/electrum
  0.89 | I | logging | Python version: 3.10.12 (main, Feb  4 2025, 14:57:36) [GCC 11.4.0]. On platform: Linux-6.8.0-52-generic-x86_64-with-glibc2.35
  0.89 | I | logging | Logging to file: /home/user/.electrum/testnet/logs/electrum_log_20250319T161247Z_6605.log
  0.89 | I | logging | Log filters: verbosity '*', verbosity_shortcuts ''
  0.89 | I | exchange_rate.FxThread | using exchange CoinGecko
  0.90 | D | util.profiler | Daemon.__init__ 0.0047 sec
  0.90 | I | daemon.Daemon | starting taskgroup.
  0.90 | I | daemon.CommandsServer | now running and listening. socktype=unix, addr=/home/user/.electrum/testnet/daemon_rpc_socket
  0.90 | I | p/plugin.Plugins | registering hardware bitbox02: ['hardware', 'bitbox02', 'BitBox02']
  0.90 | I | p/plugin.Plugins | registering hardware coldcard: ['hardware', 'coldcard', 'Coldcard Wallet']
  0.90 | I | p/plugin.Plugins | registering hardware digitalbitbox: ['hardware', 'digitalbitbox', 'Digital Bitbox wallet']
  0.90 | I | p/plugin.Plugins | could not find manifest.json of plugin hw_wallet, skipping...
  0.90 | I | p/plugin.Plugins | registering hardware jade: ['hardware', 'jade', 'Jade wallet']
  0.90 | I | p/plugin.Plugins | registering hardware keepkey: ['hardware', 'keepkey', 'KeepKey wallet']
  0.90 | I | p/plugin.Plugins | registering hardware ledger: ['hardware', 'ledger', 'Ledger wallet']
  0.90 | I | p/plugin.Plugins | registering hardware safe_t: ['hardware', 'safe_t', 'Safe-T mini wallet']
  0.90 | I | p/plugin.Plugins | registering hardware trezor: ['hardware', 'trezor', 'Trezor wallet']
  0.90 | I | p/plugin.Plugins | registering wallet type ('2fa', 'trustedcoin')
  1.01 | I | p/plugin.Plugins | loaded plugin 'labels'. (from thread: 'GUI')
  1.01 | D | util.profiler | Plugins.__init__ 0.1183 sec
```

electrum/plugin.py

@@ -61,6 +61,7 @@ plugin_loaders = {}
 hook_names = set()
 hooks = {}
 _root_permission_cache = {}
+_exec_module_failure = {}  # type: Dict[str, Exception]
 
 
 class Plugins(DaemonThread):
@@ -139,7 +140,9 @@ class Plugins(DaemonThread):
                 self.external_plugin_metadata[name] = d
 
     @staticmethod
-    def exec_module_from_spec(spec, path):
+    def exec_module_from_spec(spec, path: str):
+        if prev_fail := _exec_module_failure.get(path):
+            raise Exception(f"exec_module already failed once before, with: {prev_fail!r}")
         try:
             module = importlib.util.module_from_spec(spec)
             # sys.modules needs to be modified for relative imports to work
@@ -147,6 +150,13 @@ class Plugins(DaemonThread):
             sys.modules[path] = module
             spec.loader.exec_module(module)
         except Exception as e:
+            # We can't undo all side-effects, but we at least rm the module from sys.modules,
+            # so the import system knows it failed. If called again for the same plugin, we do not
+            # retry due to potential interactions with not-undone side-effects (e.g. plugin
+            # might have defined commands).
+            _exec_module_failure[path] = e
+            if path in sys.modules:
+                sys.modules.pop(path, None)
             raise Exception(f"Error pre-loading {path}: {repr(e)}") from e
         return module