Fix an error when rendering a SecretsError with an unknown provider to string

[glennmatthews]

Closes: #TBD

What's Changed

Fix a user-reported issue from public Slack:

   File "/usr/local/lib/python3.9/site-packages/nautobot/extras/secrets/exceptions.py", line 16, in __str__
     f'Secret "{self.secret}" (provider "{self.provider_class.__name__}"): {self.message}'
 AttributeError: 'NoneType' object has no attribute '__name__'

This happens in the case where a Secret has a configured provider string that doesn't correspond to any registered SecretsProvider class; when attempting to get the value of this Secret, a SecretProviderError is raised with a None value for its provider_class attribute; when Nautobot attempts to log this error, the SecretError.__str__() function attempts to dereference self.provider_class.__name__ which throws the above AttributeError.

To fix this issue, I've made the following changes:

• To make for slightly better string representation, allow a SecretError to store either a provider_class class reference (as before) or a raw provider string, and let the __str__() representation of the SecretError use either.
• Change the exception raised in the "no registered provider" case to pass the unknown provider string instead of None when constructing the SecretProviderError.

Additionally:

• Update the various test cases that test for SecretError to be raised so that they also now check the string representation of each such exception.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• n/a Attached Screenshots, Payload Example
• Unit, Integration Tests
• n/a Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design

[jathanism]