Differentiate stdout/stderr in CI log output

Problem to solve

Currently all log output is displayed the same, it would be great if errors (sent to stderr) could be highlighted e.g. in red.

Intended users

Developers

Further details

These errors could also be more prominently displayed in notification mails or pipeline overviews.

Proposal

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

Links / references

It probably makes sense to wait for these first:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/14664
• https://gitlab.com/gitlab-org/gitlab-ce/issues/48579