Rename GlobalSynchronization to Locks

Also address some review comments in common_throws relating to
ToStr<InvokeType> and exception detail messages.

Change-Id: Ibf2c0f147689fa236d349bd7f01eed3c2522552b
diff --git a/src/image.h b/src/image.h
index 253b762..399b143 100644
--- a/src/image.h
+++ b/src/image.h
@@ -95,7 +95,7 @@
   };
 
   Object* GetImageRoot(ImageRoot image_root) const
-      SHARED_LOCKS_REQUIRED(GlobalSynchronization::mutator_lock_) {
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     return GetImageRoots()->Get(image_root);
   }