Make squash/rebase failed messages better

What does this MR do and why?

Attempts to improve some of the messaging that happens when rebases and squashes fail in the UI.

Before:

• Failed to squash. Should be done manually
• Rebase failed. Please rebase locally. Try again.

After:

• [tbd]

Help needed from a FE

These messages should, ideally, link out to documentation to tell the user what to do next. I need assistance in adding help links to these messages.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related issues

• Related to #349397 (closed) where the issue was identified.