improve spotless error to suggest running 'gradlew tidy' (#817)

The current error isn't helpful as it suggests a per-module command. If
the user has modified multiple modules, they will be running gradle
commands to try to fix each one of them, when it would be easier to just
run 'gradlew tidy' a single time and fix everything.

gradle/validation/spotless.gradle

@@ -111,3 +111,9 @@ configure(project(":lucene").subprojects) { prj ->
     v.dependsOn ":checkJdkInternalsExportedToGradle"
   }
 }
+
+gradle.taskGraph.afterTask { Task task, TaskState state ->
+  if (task.name == 'spotlessJavaCheck' && state.failure) {
+    throw new GradleException("\n****************************\n*PLEASE RUN ./gradlew tidy!*\n****************************");
+  }
+}