patch 8.2.3975: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/gui_beval.c b/src/gui_beval.c
index 9e461e6..b460e36 100644
--- a/src/gui_beval.c
+++ b/src/gui_beval.c
@@ -103,7 +103,7 @@
 
     if (mesg != NULL && mesgCB != NULL)
     {
-	iemsg(_("E232: Cannot create BalloonEval with both message and callback"));
+	iemsg(_(e_cannot_create_ballooneval_with_both_message_and_callback));
 	return NULL;
     }