Make it easy to identify when a docker image pull fails due to bad credentials

Currently, when docker pull action fails, it produces very obscure message:

• it's not immediately obvious that actual issue is even docker pull failure as error is not handled properly, top of stacktrace actually raises error on trying to call join on nil
• it's unclear where credentials are coming from and how they are set, it requires going through actual gitlab-qa code to decipher which exact credential is at fault.

Error message should be more descriptive and not require deep dive in to implementation to be able to determine which credentials might require rotation.

Example job: https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/18096532