With #68, unsupported event types throw errors instead of failing during save. This breaks multi-event workflows that use the cache (such as a workflow that runs on schedules and push), as the scheduled build will now break due to the cache step failing.

While an if statement could be used, requiring that for a common workflow scenario can get verbose, especially as more events are added.

Example using if:

   - name: My Cache
      if: github.event_name == 'push' || github.event_name == 'pull_request'
      uses: actions/cache@v1
      with:
        path: .github
        key: ${{ runner.os }}-github

This PR changes the validation error into a warning and returns early.

Also updated to prefix warnings with a [warning] and log them normally, which will allow the highlighting in the logs without creating annotations.
Example using the [warning] prefix: