Provide static access to error names

[sfackler]

Unlike conjure-rust, it's currently hard to tell if a SerializableError is a specific Conjure error type. The error name of a Conjure error is only accessible from an instance of that type, which we don't have.

[sfackler]

I think this just looks like a breaking change to ErrorType turning fn name() into a static method and adding a few utility methods.