Commit message error is not human friendly
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### Summary
*Require expression in commit messages* in *Push rules* doesn't allow for a custom error message.
The error message is a regex pattern and that is not user-friendly.
We would like to write our own error message to explain what's wrong and provide examples.
### Steps to reproduce
1. Set-up a regex pattern in *Require expression in commit messages* in *Push rules*. For example, `^(?:build|chore|ci|docs|feat|fix|perf|refactor|revert|style|test)(?:\([a-zA-Z->]*\))?: `

2. Try to commit with a commit message that doesn't comply with the regex pattern.
3. You should see an error message that says **Unexpected error** and then states the regex pattern.

### Example Project
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behavior, and link to it here in the bug report. If you are using an older version of GitLab, this
will also determine whether the bug is fixed in a more recent version. -->
### What is the current *bug* behavior?
The error message for non-compliant commit message is not editable and is not user-friendly by default.
### What is the expected *correct* behavior?
The error message is editable or user-friendly by default.
### Relevant logs and/or screenshots

### Output of checks
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#### Results of GitLab environment info
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<details>
<summary>Expand for output related to GitLab environment info</summary>
<pre>
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)
(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)
</pre>
</details>
#### Results of GitLab application Check
<!-- Input any relevant GitLab application check information if needed. -->
<details>
<summary>Expand for output related to the GitLab application check</summary>
<pre>
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:check SANITIZE=true`)
(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)
(we will only investigate if the tests are passing)
</pre>
</details>
### Possible fixes
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issue