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/ex_eval.h

@@ -40,8 +40,8 @@
  * message in the list.  See cause_errthrow() below.
  */
 struct msglist {
-  char_u *msg;             // original message
-  char_u *throw_msg;       // msg to throw: usually original one
+  char *msg;             // original message
+  char *throw_msg;       // msg to throw: usually original one
   struct msglist *next;            // next of several messages in a row
 };
 
@@ -60,7 +60,7 @@ typedef enum
 typedef struct vim_exception except_T;
 struct vim_exception {
   except_type_T type;                   // exception type
-  char_u *value;           // exception value
+  char *value;           // exception value
   struct msglist *messages;        // message(s) causing error exception
   char_u *throw_name;      // name of the throw point
   linenr_T throw_lnum;                  // line number of the throw point