Error codes for Accounts constraints

[armaniferrante]

The DSL is currently very lazy with error codes. We should do this properly, generating an error for each constraint, adding those to the IDL, and having all clients report typed, informative errors, when the runtime responds with a particular error code.

There's also some boilierplate we should eliminate with a macro when creating custom errors for programs.

[armaniferrante]

Part of this is addressed by #25. Namely, the ability to define custom, typed errors that propagate to the client.

The other part needed to be done is informative errors for the constraints in the #[derive(Accounts)] struct. Right now, we just return a bunch of useless ProgramError::Custom(1) errors.

[armaniferrante]

Completed by #354.