ErrorProne: Increase severity of FutureReturnValueIgnored to ERROR

The only remaining code where the return value is ignored is in tests.
Update them to store the value and perform a basic assertion.

Change-Id: I29ef5bd5dd0648aac3490f9e47ecc74544109652
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.lfs.server.test/tst/org/eclipse/jgit/lfs/server/fs/UploadTest.java

@@ -44,6 +44,7 @@ package org.eclipse.jgit.lfs.server.fs;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
@@ -56,6 +57,7 @@ import java.util.List;
 import java.util.concurrent.CyclicBarrier;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 
 import org.eclipse.jgit.lfs.lib.AnyLongObjectId;
@@ -122,11 +124,12 @@ public class UploadTest extends LfsServerTest {
 		ExecutorService e = Executors.newFixedThreadPool(count);
 		try {
 			for (Path p : paths) {
-				e.submit(() -> {
+				Future<Object> result = e.submit(() -> {
 					barrier.await();
 					putContent(p);
 					return null;
 				});
+				assertNotNull(result);
 			}
 		} finally {
 			e.shutdown();

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcPackRefsTest.java

@@ -176,12 +176,13 @@ public class GcPackRefsTest extends GcTestCase {
 				return update.update();
 			});
 
-			pool.submit(() -> {
+			Future<Result> result2 = pool.submit(() -> {
 				refUpdateLockedRef.await();
 				gc.packRefs();
 				packRefsDone.await();
 				return null;
 			});
+			assertNull(result2.get());
 
 			assertSame(result.get(), Result.FORCED);
 

tools/BUILD

@@ -42,7 +42,7 @@ java_package_configuration(
         "-Xep:FragmentInjection:WARN",
         "-Xep:FragmentNotInstantiable:WARN",
         "-Xep:FunctionalInterfaceClash:WARN",
-        "-Xep:FutureReturnValueIgnored:WARN",
+        "-Xep:FutureReturnValueIgnored:ERROR",
         "-Xep:GetClassOnEnum:WARN",
         "-Xep:ImmutableAnnotationChecker:WARN",
         "-Xep:ImmutableEnumChecker:WARN",