Move label check failure to mergebot (#94707)
Fixes https://github.com/pytorch/pytorch/issues/88098
This is a mirror of the same PR (https://github.com/Goldspear/pytorch/pull/3) that has been reviewed in my fork (due to it's a stacked PR).
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## Context
This the 3rd of the 3 PRs to address the issue 88098.
## What Changed
1. check_labels.py no longer fails, but only leaving a comment
2. trymerge.py now would fail if no required labels provided
## Tests
* dummy-repo trymerge run [fails without required label](https://github.com/Goldspear/pytorch-dummy/actions/runs/4162819216) and resulted in [a label error comment](https://github.com/Goldspear/pytorch-dummy/pull/3#issuecomment-1427756769)
* the above pr was [correctly merged](https://github.com/Goldspear/pytorch-dummy/pull/3) after label is added.
## Note to Reviewers
1st PR: https://github.com/pytorch/pytorch/pull/94179
2nd PR: https://github.com/pytorch/pytorch/pull/94899
3rd PR: this one
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94707
Approved by: https://github.com/ZainRizvi
