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

src/gradlew.ts

@@ -1,3 +1,6 @@
+import * as path from 'path'
+import fs from 'fs'
+
 const IS_WINDOWS = process.platform === 'win32'
 
 export function wrapperFilename(): string {
@@ -7,3 +10,15 @@ export function wrapperFilename(): string {
 export function installScriptFilename(): string {
     return IS_WINDOWS ? 'gradle.bat' : 'gradle'
 }
+
+export function validateGradleWrapper(gradlewDirectory: string): void {
+    const wrapperProperties = path.resolve(
+        gradlewDirectory,
+        'gradle/wrapper/gradle-wrapper.properties'
+    )
+    if (!fs.existsSync(wrapperProperties)) {
+        throw new Error(
+            `Cannot locate a Gradle wrapper properties file at '${wrapperProperties}'. Specify 'gradle-version' or 'gradle-executable' for projects without Gradle wrapper configured.`
+        )
+    }
+}

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())