fido2.client: Raise unhandled error in cred_mgmt

[robin-nitrokey]

An error with an unexpected error code was previously ignored in NKFido2Client.cred_mgmt, leading to an uninitialized variable being used in the following code. This patch fixes the issue by raising all unhandled errors.

Fixes: https://github.com/Nitrokey/pynitrokey/issues/355

Checklist

• [x] tested with Python3.9
• [x] run make check or make fix for the formatting check
• [x] signed commits
• [ ] updated documentation (e.g. parameter description, inline doc, docs.nitrokey)
• [x] added labels

[szszszsz]

The source error does not have any local_prints though

[szszszsz]

Must have been something else than CtapError.ERR.*, but raise should now make it a friendly user message, should it not?

Edit: it does look so - it has _str_ method