Incorrectly logging error with unavailable RHSM certificate

Introduced in https://github.com/oamg/convert2rhel/pull/203

Reproduce:

1. Run unsigned convert2rhel (e.g. from copr) without the -y option
2. Answer "n" to the first question if we want to continue (that's under the step "Convert: List third-party packages")
3. Observe rollback error:

[01/04/2022 11:16:35] TASK - [Rollback: Removing installed RHSM certificate] ********************
ERROR - OSError(2): No such file or directory

Acceptance criteria:

• We don't log an error when the RHSM cert is not installed during rollback as it might easily just be that convert2rhel hasn't got to installing it.
• An integration test is created