CLI: add_request breaks without digits as first param

$ electrum add_rrequest help

command will output:

Traceback (most recent call last):
  File "/usr/local/bin/electrum", line 472, in <module>
    main()
  File "/usr/local/bin/electrum", line 311, in main
    args = parser.parse_args()
  File "/usr/lib/python3.7/argparse.py", line 1758, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib/python3.7/argparse.py", line 1790, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python3.7/argparse.py", line 1999, in _parse_known_args
    stop_index = consume_positionals(start_index)
  File "/usr/lib/python3.7/argparse.py", line 1955, in consume_positionals
    take_action(action, args)
  File "/usr/lib/python3.7/argparse.py", line 1864, in take_action
    action(self, namespace, argument_values, option_string)
  File "/usr/local/lib/python3.7/dist-packages/electrum/commands.py", line 1392, in subparser_call
    namespace, arg_strings = parser.parse_known_args(arg_strings, namespace)
  File "/usr/lib/python3.7/argparse.py", line 1790, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python3.7/argparse.py", line 1999, in _parse_known_args
    stop_index = consume_positionals(start_index)
  File "/usr/lib/python3.7/argparse.py", line 1955, in consume_positionals
    take_action(action, args)
  File "/usr/lib/python3.7/argparse.py", line 1848, in take_action
    argument_values = self._get_values(action, argument_strings)
  File "/usr/lib/python3.7/argparse.py", line 2378, in _get_values
    value = self._get_value(action, arg_string)
  File "/usr/lib/python3.7/argparse.py", line 2411, in _get_value
    result = type_func(arg_string)
  File "/usr/local/lib/python3.7/dist-packages/electrum/commands.py", line 1325, in <lambda>
    'amount': lambda x: str(Decimal(x)) if x != '!' else '!',
decimal.InvalidOperation: [<class 'decimal.ConversionSyntax'>]

Start by reproducing the reported electrum add_rrequest help failure, then inspect the argument parsing code in electrum/commands.py around the amount conversion shown in the traceback. Check how invalid positional input is handled elsewhere in the CLI. Done means this input produces a user-facing command error rather than an unhandled traceback.