Fix/issue64 - added fallthrough in error handling

[mathiaswegner]

The boto3 call to get the secret value is wrapped in a try/except. The except block has a switch statement to raise different exceptions based on the error content. There was no fallthrough for the switch, so errors that were not explicitly defined were masked behind a subsequent error about the secret_value being referenced before it was assigned.

[glennmatthews]

CI is failing unrelated to your changes, we need to bring in the fix from https://github.com/nautobot/cookiecutter-nautobot-app/pull/111.