introduce UserFacingException

we should not raise generic Exception when wanting to communicate with
the user. it makes distinguishing programming errors and messages hard,
as the caller will necessarily need to catch all Exceptions then

electrum/util.py

@@ -119,6 +119,10 @@ class WalletFileException(Exception): pass
 class BitcoinException(Exception): pass
 
 
+class UserFacingException(Exception):
+    """Exception that contains information intended to be shown to the user."""
+
+
 # Throw this exception to unwind the stack like when an error occurs.
 # However unlike other exceptions the user won't be informed.
 class UserCancelled(Exception):