Add information to unimplemented! messages

[fprasx]

The first time I ran trustfall_stubgen this is what I saw:

thread 'main' panicked at~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/trustfall_core-0.7.1/src/schema/mod.rs:163:46:
not implemented
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This should alleviate that a bit. At some point, we should also have a reference detailing the difference between Trustfall and GraphQl schemas.

[obi1kenobi]

Thanks! Yeah, all of this is stuff we should support someday but isn't a high priority right now, since Trustfall is quite usable without it.

[fprasx]

Pushed a format, should pass now