Add failure reason for execution timeout

What does this MR do?

• GitLab-rails: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22224
• GitLab-runner: here

The copy is The script exceeded the maximum execution time set for the job.

Why was this MR needed?

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?