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.

.github/workflows/prod.yml

@@ -46,3 +46,15 @@ jobs:
         gradle-executable: __tests__/samples/basic/gradlew${{ matrix.script-suffix }}
         build-root-directory: __tests__/samples/no-wrapper
         arguments: help
+
+  failures: # These build invocations are informational only, and are expected to fail
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@v2
+    - name: Test wrapper missing
+      uses: ./
+      continue-on-error: true
+      with:
+        build-root-directory: __tests__/samples/no-wrapper
+        arguments: help