Include identifier in error message when finding non-alphanumeric chars

[dfaust]

Small improvement that makes it more obvious where the cause of the error lies.

[dwrensha]

This change looks good to me, but I'm curious: did you manage to trigger these errors when compiling a capnp schema? I would expect capnp compile to catch the non-alphanumeric identifiers before passing them to capnpc-rust.

[dfaust]

Yes, I hit the one in camel_to_snake_case because I had an argument for a method formatted in snake case instead of camel case.

[dwrensha]

Interesting! I've opened as issue upstream: https://github.com/capnproto/capnproto/issues/506

[dwrensha]

Thanks!