Fix output and exit code for java tests

Before this change, run_java_unit_tests.js would always have an exit
code of 0 thus never failing the npm tests.
Furthermore, "Tests completed successfully" would even be printed after
failing to create the Gradle wrapper.

test/run_java_unit_tests.js

@@ -37,11 +37,14 @@ if (!needs_gradlew_built) {
 }
 
 needs_gradlew_built.then(function () {
-    return superspawn.spawn(path.join(__dirname, 'gradlew'), ['test'], {stdio: 'inherit'});
+    return superspawn.spawn(path.join(__dirname, 'gradlew'), ['test'], {stdio: 'inherit'})
+        .then(function () {
+            console.log('Tests completed successfully.');
+        }, function (err) {
+            console.error('Tests failed!', err);
+            process.exitCode = 1;
+        });
 }, function (err) {
     console.error('There was an error building the gradlew file:', err);
-}).then(function () {
-    console.log('Tests completed successfully.');
-}).fail(function (err) {
-    console.error('Tests failed!', err);
+    process.exitCode = 1;
 });