Define errors_by_code in ipalib.errors

[frasertweedale]

The errors_by_code mapping will soon be used in more places, as part
of the Dogtag GSS-API authentication work. Move its definition to
ipalib.errors.

Part of: https://pagure.io/freeipa/issue/5011

[HonzaCholasta]

The codes are only relevant for XML-RPC. Why do you need them?

[frasertweedale]

@HonzaCholasta when Dogtag execute the existing cert-request validation logic (which will be extracted to a new function), if an exception gets raised Dogtag returns it in the response, and IPA reconstructs it, so that there is no change to the user experience.

Pickling is an alternative.

[HonzaCholasta]

Seems like an overkill for the ~2 types of errors which the code uses. Anyway, I would rather wait before making any kind of decision based on cert-request code until after it is refactored to be less insane (https://pagure.io/freeipa/issue/6531).

[frasertweedale]

Righto. I'll withdraw this PR for now and it will make a comeback closer to landing the gssapi work.