Finer-grained parser errors

[hgzimmerman]

The parser subsystem now supports reporting errors inside of capture groups.

This looks like:

   = help: message: Invalid Matcher: Could not parse route.
           Route: /bad_route{4hello}
           ------------------^
           Expected: <ident>
           Reason: The character: '4' could not be used as a Rust identifier.

To enable this functionality, the core parsers now report errors themselves: closes #149