Fix gradle.py to only raise if there is actually a problem

R=ricow@google.com

Bug:
Change-Id: I744f72338e7f3de2b24f8e9cc7b6e32ab497b037
diff --git a/tools/gradle.py b/tools/gradle.py
index 3a309ab..488dc87 100755
--- a/tools/gradle.py
+++ b/tools/gradle.py
@@ -40,7 +40,7 @@
   utils.PrintCmd(cmd)
   with utils.ChangedWorkingDirectory(utils.REPO_ROOT):
     return_value = subprocess.call(cmd)
-    if throw_on_failure:
+    if throw_on_failure and return_value != 0:
       raise
     return return_value