Use custom exception type for SSL issues

[robin-nitrokey]

Currently, we have some awkward code in pynitrokey to dectect self-signed SSL certificates:

        try:
            yield nethsm
        except nethsm_sdk.NetHSMError as e:
            raise click.ClickException(e)
        except urllib3.exceptions.MaxRetryError as e:
            if isinstance(e.reason, urllib3.exceptions.SSLError):
                raise click.ClickException(
                    f"Could not connect to the NetHSM: {e.reason}\nIf you use a self-signed certificate, please set the --no-verify-tls option."
                )
            else:
                raise e

It would be nice if we would not have to assume that urllib3 is used and could just check some NetHSM exception types. The urllib3 exceptions could be wrapped in NetHSMError instances and set a special flag if it is an SSL error, or a new exception could be introduced for SSL errors.