Give better error message when merge fails to find any rules (#84160)
Fixes #84147 and https://github.com/pytorch/test-infra/issues/421.
* If merge rule file is missing or fails to load for whatever reasons:
```
No rules find to match PR, please [report]{issue_link} this issue to DevX team.
```
* If the list of rules is empty:
```
Merges are not allowed into repository without a rules.
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84160
Approved by: https://github.com/ZainRizvi, https://github.com/malfet
