[REFACTOR] Align Exception classes between Artpiece and User API #104
Description
They way exception classes work in web.api.user.exceptions is structurally different from the way they work in web.api.user.artpiece.excpetions. The latter module inherits the InvalidUsage class from the former and defines subclasses, while the former just adds new methods to the InvalidUsage class.
These should structurally work the same way for consistency of usage.