Wrap errors raised by dec_hook in ValidationError #456

Description

Currently if dec_hook raises an error, the error is raised as is by the corresponding decode call. This is the only place where a msgspec.ValidationError or msgspec.DecodeError isn't raised by decode. To fix this, we should:

• Update the docs to recommend raising NotImplementedError in a dec_hook when a type isn't handled by the dec_hook
• wrap any TypeError or ValueError raised by dec_hook in a ValidationError with additional context. Ideally we could also used chained exceptions here, although the c-api makes handling this a bit tricky.