synchronizer: allow server not finding txn sometimes

User has wallet file with history that includes some txid; corresponding
raw tx is not in the "transactions" dict in the file however.
When the synchronizer starts up, it requests this "missing" txn from
the server... but what if the server does not know about it?
Maybe it was reorged and is not in the new best chain,
and not even in mempool. This was not handled previously.

fix #5122

electrum/network.py

@@ -780,7 +780,7 @@ class Network(PrintError):
             try:
                 return await func(self, *args, **kwargs)
             except aiorpcx.jsonrpc.CodeMessageError as e:
-                raise UntrustedServerReturnedError(original_exception=e)
+                raise UntrustedServerReturnedError(original_exception=e) from e
         return wrapper
 
     @best_effort_reliable