Make terraform fmt more verbose

The following discussion from !69470 (merged) should be addressed:

• @marcel.amirault started a discussion: (+2 comments)

  @willianpaixao Thanks a lot for the contribution! I agree with @Alexand that this looks good!

  As a sidenote, I really wish fmt had a bit more verbose output. Something like "These files are not formatted:" or "All checked files are formatted." (but that's outside the scope of this MR). @Alexand, do you think we should raise a follow-up issue to write a little script that makes the output nicer, or will the average user of this template be OK with the "dry" output?

  Also, thanks a lot for the test pipeline links, the fail/pass examples were just what I needed! 🙇

Proposed solutions

1. Simply add the -diff flag.

   # lib/gitlab/ci/templates/Terraform/Base.latest.gitlab-ci.yml
   - - gitlab-terraform fmt -check -recursive
   + - gitlab-terraform fmt -check -recursive -diff

2. (optional) collect the exit code of the terraform fmt... command and echo a message explaining how to fix it. Something like:

To fix your terraform format, run locally: terraform fmt -recursive