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

[Samarth Narang via cfe-commits]

snarang181 wrote:

@ilovepi, thanks for the patience and taking out the time to give detailed reviews. I will work on addresing your comments. One word on the tests: I was trying to test some of the other fail conditions but seems like a lot of the error conditions are caught by the parsing early on and do not actually invoke our error handler (like the current existing test case). Would adding those cases be OK regardless? Even though this patch does not directly address those issues. In that vein, it might be better to add `clang-doc` tests as part of a different PR, and I will be happy to work on that follow up too. Let me know what you think. 

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