parse_as gives a bad error if it failed to parse the message

[museun]

e.g.

let input = "@reply-parent-msg-id=test_msg_id PRIVMSG #test :usage: !hello <world>\r\n";
let pm = parse_as::<Privmsg>(input).unwrap();

it'll give an error like:
IncorrectMessageType { expected: "Privmsg", got: "Privmsg" }

when its actually a bad parse ("missing the 'prefix' -- which it should handle. but thats another issue)

[museun]

parse_as() would need to return a Result<T, T::Err> for this to happen

currently it just returns an Option<T>