daemon: refactor load_wallet to not just return None, but raise specific exceptions.

The following exceptions should be expected:
FileNotFoundError: given wallet path does not exist
StorageReadWriteError: given file is not readable/writable or containing folder is not writable
InvalidPassword: wallet requires a password but no password or an invalid password was given
WalletFileException: any internal wallet data issue. specific subclasses can be caught separately:
-  WalletRequiresSplit: wallet needs splitting (split_data passed in Exception)
-  WalletRequiresUpgrade: wallet needs upgrade, and no upgrade=True was passed to load_wallet
-  WalletUnfinished: wallet file contains an action and needs additional information to finalize. (WalletDB passed in exception)

Removed qml/qewalletdb.py

This patch also fixes load_wallet calls in electrum/scripts and adds a qml workaround for dialogs opening and closing so
fast that the dialog opened==true property change is missed (which we need to manage the dialog/page stack)

electrum/gui/qml/components/main.qml

@@ -498,17 +498,21 @@ ApplicationWindow
 
     property var _opendialog: undefined
 
+    function showOpenWalletDialog(name, path) {
+        if (_opendialog == undefined) {
+            _opendialog = openWalletDialog.createObject(app, { name: name, path: path })
+            _opendialog.closed.connect(function() {
+                _opendialog = undefined
+            })
+            _opendialog.open()
+        }
+    }
+
     Connections {
         target: Daemon
         function onWalletRequiresPassword(name, path) {
             console.log('wallet requires password')
-            if (_opendialog == undefined) {
-                _opendialog = openWalletDialog.createObject(app, { path: path, name: name })
-                _opendialog.closed.connect(function() {
-                    _opendialog = undefined
-                })
-                _opendialog.open()
-            }
+            showOpenWalletDialog(name, path)
         }
         function onWalletOpenError(error) {
             console.log('wallet open error')