[clang-tools-extra] [clang-doc] Refactor error handling to use ExitOnError (PR #141699)

[Paul Kirth via cfe-commits]

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

I think this will be fine to land after the small changes I've requested are done, and CI is passing. Thanks for helping out clang-doc, and bearing with the review process.

https://github.com/llvm/llvm-project/pull/141699