vim-patch:8.1.0743: giving error messages is not flexible

Problem:    Giving error messages is not flexible.
Solution:   Add semsg().  Change argument from "char_u *" to "char *", also
            for msg() and get rid of most MSG macros. (Ozaki Kiichi, closes
            vim/vim#3302)  Also make emsg() accept a "char *" argument.  Get rid of
            an enormous number of type casts.
f9e3e09fdc

src/nvim/eval/executor.c

@@ -144,6 +144,6 @@ int eexe_mod_op(typval_T *const tv1, const typval_T *const tv2, const char *cons
     }
   }
 
-  EMSG2(_(e_letwrong), op);
+  semsg(_(e_letwrong), op);
   return FAIL;
 }