Minor: Add help message on CI failure (#182)

**What does this PR do?**:

Add a tiny message pointing people in the right direction if something
fails in GitLab CI.

**Motivation**:

I got confused by this, so might as well improve it so that I'm the
last person to be confused :)

**Additional Notes**:

N/A

**How to test the change?**:

Validate that this message shows up when going to the
"report_gitlab_CI_status" job in CI.

.gitlab-ci.yml

@@ -36,4 +36,5 @@ report_gitlab_CI_status:
   when: always
   stage: .post
   script:
+    - echo "TIP: If you see this failing, it's because something else in the GitLab pipeline failed. Follow the link to the pipeline and check for other things that failed prior to this step."
     - exit ${STATUS}