allow paying invoice without amount. min feerate 253 sat/kw.

electrum/util.py

@@ -131,6 +131,7 @@ class BitcoinException(Exception): pass
 class UserFacingException(Exception):
     """Exception that contains information intended to be shown to the user."""
 
+class InvoiceError(Exception): pass
 
 # Throw this exception to unwind the stack like when an error occurs.
 # However unlike other exceptions the user won't be informed.