bless_test_results: be a bit more forgiving about return code

[jgfouca]

Description

If the user tells BTR to bless everything it can, and BTR encounters a FAILed test that did not DIFF, that should not result in the overall BTR run being a failure; just skip that test. It should only be an overall failure if the user specifically requested the bless of that test. This behavior is necessary for our auto blesser to work correctly.

Checklist

• My code follows the style guidelines of this project (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that exercise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation