vim-patch:8.2.0035: saving and restoring called_emsg is clumsy (#19335)

Problem:    Saving and restoring called_emsg is clumsy.
Solution:   Count the number of error messages.
53989554a4

src/nvim/message.c

@@ -637,7 +637,7 @@ static bool emsg_multiline(const char *s, bool multiline)
     return true;
   }
 
-  called_emsg = true;
+  called_emsg++;
 
   // If "emsg_severe" is true: When an error exception is to be thrown,
   // prefer this message over previous messages for the same command.