[clang] [clang-repl] Improve error message on failed undos (PR #149396)

[Aaron Danen via cfe-commits]

aadanen wrote:

@DavidSpickett I applied clang-format and I think everything is looking good. Do I need to do anything else? Or is squashing the commits and merging up to someone with write permissions?

Also, thanks a lot for the help. I plan to find a new issue to work on soon. Thanks for offering to help there as well! I bet it will save me a lot of time and confusion.

https://github.com/llvm/llvm-project/pull/149396