[6.1.0]Improve error message for concurrent modification

[ShreeM01]

Improve the error message logged when concurrent modifications occur and --experimental_guard_against_concurrent_changes is set.

Closes #17174.

PiperOrigin-RevId: 501012999
Change-Id: Id661ff2e5f98173323ebecb75b5e9825c8e0491e

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[tbaing]

It wasn't obvious to me how to rebase this PR to pick up my email address after updating my settings to show it in commit messages, so I created #17350 to replace this one. So that one can be merged and this one abandoned.

[tbaing]

Would it be best to update the author email in the cherrypick commit here to use my Google email ([email protected])? I don't think I've got permissions to do that...

[keertk]

Closing - see #17396