Treat git commands as failed if output has errors

What does this MR do?

The test framework is supposed to retry a push if it fails, but if there's a problem that's reported as a warning, it's possible for the git command to still be treated as successful.

This makes it fail if the output reports that an error was encountered.

It also shows something more informative than undefined method '[]' for nil:NilClass if there's no MR created.

See #254951 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team