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

[David Spickett via cfe-commits]

https://github.com/DavidSpickett approved this pull request.

LGTM

@aadanen you won't be able to merge this yourself but I can once you've fixed the formatting.

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