Improve reasoning for MR creation emails rejection

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If you submit invalid input when creating MRs via email all the rejections you receive will have this error message:

Unfortunately, your email message to GitLab could not be processed.

You are not allowed to perform this action. If you believe this is in error, contact a staff member.

This goes for every reason your attempts was rejected: invalid branch name, no permission, etc.

We should improve the message to have a bit more clarity on the rejection reasoning.

Following-up on a discussion:

I'm adding a troubleshooting item for this outcome in !139598 (32de5650). I wonder if we should improve this email message:

• app/views/email_rejection_mailer/rejection.html.haml
• app/views/email_rejection_mailer/rejection.text.haml