oc: Use enum mapistore_error instead of int

This specifies a little the scope of the variable to make it
more realistic with the actual values it may have. We do have
a static typed compiled language, why don't we use it?

OpenChange/MAPIStoreMailVolatileMessage.h

@@ -27,7 +27,7 @@
 
 @interface MAPIStoreMailVolatileMessage : MAPIStoreMessage
 
-- (int) submitWithFlags: (enum SubmitFlags) flags;
+- (enum mapistore_error) submitWithFlags: (enum SubmitFlags) flags;
 
 @end