Check executable bit on Gradle script

Fixes #76

src/execution.ts

@@ -14,6 +14,7 @@ export async function executeGradleBuild(executable: string | undefined, root: s
 
     // Use the provided executable, or look for a Gradle wrapper script to run
     const toExecute = executable ?? gradlew.locateGradleWrapperScript(root)
+    verifyIsExecutableScript(toExecute)
     const status: number = await exec.exec(toExecute, args, {
         cwd: root,
         ignoreReturnCode: true
@@ -31,3 +32,11 @@ export async function executeGradleBuild(executable: string | undefined, root: s
         }
     }
 }
+
+function verifyIsExecutableScript(toExecute: string): void {
+    try {
+        fs.accessSync(toExecute, fs.constants.X_OK)
+    } catch (err) {
+        throw new Error(`Gradle script '${toExecute}' is not executable.`)
+    }
+}