lang: Framework defined error codes

[armaniferrante]

Completes #9.

[ruuda]

I believe this change broke conversion from ProgramError back to ErrorCode:

• Conversion to ProgramError adds the offset here: https://github.com/project-serum/anchor/blob/ba99c9c920c9a7924c63fc1d1af9a7316e9d6053/lang/syn/src/codegen/error.rs#L78
• But conversion back from ProgramError uses a derived From impl, which does not subtract the offset first: https://github.com/project-serum/anchor/blob/ba99c9c920c9a7924c63fc1d1af9a7316e9d6053/lang/syn/src/codegen/error.rs#L57