Use MonadError for SourceParsers about documentalist

[mxswd]

Can these actually error? It sounds like we are just return a Nothing for parts that can't parse. Which is fine.

[jspahrsummers]

I think we'll eventually want to report errors (in case they're legitimate syntactical issues), though they should probably be considered non-fatal.

[jspahrsummers]

Eh. Decided that exceptions are okay for what we're doing.

[mxswd]

Yeah. I agree. I think parseDocs though should change from:
parseDocs :: p -> Package (Maybe Comment) -> Either CommentParseException (Package (Maybe DocBlock))
to:
parseDocs :: p -> Package (Maybe Comment) -> Package (Either CommentParseException DocBlock)
Because a Package or Module or Declaration can't "fail". Only the Maybe Comment.

Making a new PR now.

[mxswd]

I would be keen on just parseDocs :: p -> Package (Maybe Comment) -> Package (Maybe DocBlock) instead as well.

[mxswd]

See issue #38