Introduce ciel-specific Python exceptions

Description

Ciel currently raises normal ValueErrors, RuntimeErrors, etc. This is theoretically fine. but practically that means we have to capture all exceptions for purposes of presenting them to the users, which can include exceptions in other libraries and/or the Python standard library, and not show a traceback. This is a pretty poor user/developer experience.

Proposal

New exception classes should inherit from ValueErrors/RuntimeErrors to match the declared APIs, but ciel should raise exclusively these classes.