chore: make release build trigger more resilient

1. fail early if tag is created, e.g. release@18.0.44-alpha, or branch is created, e.g. auto/version-master-18.0.45 (report on both at failure!)
2. Avoid adding comments multiple times to each of the relevant PRs

Back-story:
* https://build.palaso.org/buildConfiguration/Keyman_TriggerReleaseBuildsMaster/466404?buildTab=log&linesState=257&logView=flowAware failed:

```
01:06:06  TeamCity Build Command:
01:06:06 
01:06:06  Process exited with code 60
```

* https://build.palaso.org/buildConfiguration/Keyman_TriggerReleaseBuildsMaster/468111?buildTab=log&linesState=222&logView=flowAware&focusLine=404 and friends failed:

```
02:21:34  increment-version.sh: committing to repository and tagging release version 18.0.45-alpha
02:21:34  fatal: tag 'release@18.0.45-alpha' already exists
```

* And finally, when I re-ran manually, I forgot to delete the branch, and https://build.palaso.org/buildConfiguration/Keyman_TriggerReleaseBuildsMaster/468136?buildTab=log&focusLine=373&logView=linear&linesState=191 failed:

```
06:07:01  # New version is 18.0.46
06:07:01 
06:07:01  increment-version.sh: committing to repository and tagging release version 18.0.45-alpha
06:07:01  fatal: A branch named 'auto/version-master-18.0.46' already exists.
```

Ref also https://github.com/keymanapp/keyman/wiki/HOWTO:-cleanup-if-a-release-build-trigger-fails