on some hook events, uc_hook_add() should not allow more than 1 handler. this add UC_ERR_HOOK_EXIST error type

include/unicorn/unicorn.h

@@ -124,6 +124,7 @@ typedef enum uc_err {
     UC_ERR_READ_UNALIGNED,  // Unaligned read
     UC_ERR_WRITE_UNALIGNED,  // Unaligned write
     UC_ERR_FETCH_UNALIGNED,  // Unaligned fetch
+    UC_ERR_HOOK_EXIST,  // hook for this event already existed
 } uc_err;