Validate presense of gradle wrapper to provide better feedback

- Provide a more useful error message when no Gradle wrapper can be located,
  and 'gradle-version' or 'gradle-executable' is not used.
- Add test for case where wrapper is missing.
  This isn't really a "test" per-se, but this failing build invocation makes it
  easy to verify the GitHub action behaviour when the build is misconfigured.

src/main.ts

@@ -62,6 +62,7 @@ async function resolveGradleExecutable(
             ? path.resolve(workspaceDirectory, wrapperDirectory)
             : buildRootDirectory
 
+    gradlew.validateGradleWrapper(gradlewDirectory)
     await cacheWrapper.restoreCachedWrapperDist(gradlewDirectory)
 
     return path.resolve(gradlewDirectory, gradlew.wrapperFilename())