[Sync] Use ModelError all the way through to ModelTypeController.

components/sync/model/model_error.h

Index: components/sync/model/model_error.h
diff --git a/components/sync/model/model_error.h b/components/sync/model/model_error.h
index e61816da37c1bd82802601466aaa321936c04ce4..ebd0dae57076d0b5e2c5fb5b5fd90204edd6e3d9 100644
--- a/components/sync/model/model_error.h
+++ b/components/sync/model/model_error.h
@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/callback.h"
 #include "base/location.h"
 
 namespace syncer {
@@ -40,6 +41,9 @@ class ModelError {
   std::string message_;
 };
 
+// Typedef for a simple error handler callback.
+using ModelErrorHandler = base::RepeatingCallback<void(const ModelError&)>;
+
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_MODEL_MODEL_ERROR_H_