anchor: add custom error for raw constraint

[fanatid]

Issue #509

• Should we add custom error support for all constraints?
• Is there a way to allow custom errors for Signer / Program?
• We can't use ? because it's parsed with expr, are we ok with @?

[armaniferrante]

LGTM.

• Probably, though happy to start with constraint = only for now.
• No, but we can solve that by mapping the error and logging the context (e.g. the variable name and key)
• @ sounds good to me

[fanatid]

Thanks. I think I need to add some documentation and lines to CHANGELOG.md before merging.

[fanatid]

Updated. I also moved errors-related code to functions, so adding custom errors for other constraints should be easy.