Allow parsers to output Messages

[brynary]

We have a data transfer object called Message which is for diagnostics. For example, we output a Message of level error if a plugin returns too many issues and we cut them off.

However, a trait Parser today returns Result<Vec<Issue>>, so it is not possible to emit Messages from the parser itself. We should extend the Parser interface to return a new struct ParserResult which has both a Vec<Issue> and Vec<Message>