patch 8.2.3959: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more errors to errors.h.
diff --git a/src/gui.c b/src/gui.c
index 9b19cef..a5f449a 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -388,7 +388,7 @@
     if (result != MAYBE)
     {
 	if (result == FAIL)
-	    emsg(_("E229: Cannot start the GUI"));
+	    emsg(_(e_cannot_start_the_GUI));
 	return result;
     }