ra: Add error value & conditions to NewRegistrationAuthorityImpl

[jprenken]

ra.NewRegistrationAuthorityImpl takes some arguments that are pointers, and could be nil (e.g. limiter). It currently does not return an error. We should add an error value, return errors when passed nils for these arguments, and update calling functions to handle errors.

Discussed in: https://github.com/letsencrypt/boulder/pull/7858/files#r1906117221