patch 8.2.4012: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move the last error messages to errors.h.
diff --git a/src/gui_w32.c b/src/gui_w32.c
index cba3179..0f9a20e 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -4991,7 +4991,7 @@
     static char *
 gvim_error(void)
 {
-    char *msg = _("E988: GUI cannot be used. Cannot execute gvim.exe.");
+    char *msg = _(e_gui_cannot_be_used_cannot_execute_gvim_exe);
 
     if (starting)
     {