Allow command parsing to fail without throwing an exception

[FiniteReality]

It's likely that in a bot for, say, IRC or Discord, will be listening to all messages, and then attempting to parse them. This means that if a parser throws an exception when a command fails to parse, it will be throwing on every message. Since failing to parse is, in this case, not an exceptional error, I should put some work in to make sure that parsers can communicate failure without throwing an exception.