Report useful error when no files at all exist

[JLLeitschuh]

If this action is run and a user forgets to add the - uses: actions/checkout@v2 they get this uninformative error:

Run gradle/wrapper-validation-action@v1
  with:
    wrapper-directory: gradle/wrapper
    gradle-executable: gradlew
    allow-snapshots: true
    min-wrapper-count: 1
##[error]Gradle Wrapper Validation Failed!
  See https://github.com/gradle/wrapper-validation-action#reporting-failures
✗ Other validation errors:
  Expected to find at least 1 Gradle Wrapper JARs but got only 0

We should provide a more useful error when the GitHub action is running in the context of no files at all so that there is a more actionable step that users can take.

[akuleshov7]

For sure! it is very confusing, I was thinking for several days that the problem is with github actions

[pu6karev]

I have marked jar files in gitignore earlier. I removed this file type from gitignore and everything worked. Of course, added the specific gradle-wrapper.jar to repo.