patch 8.2.3190: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move error messages to errors.h and give them a clear name.
diff --git a/src/gui.c b/src/gui.c
index c748fae..687e9da 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5038,7 +5038,7 @@
 #if defined(VIMDLL) && !defined(EXPERIMENTAL_GUI_CMD)
 	if (!gui.starting)
 	{
-	    emsg(_(e_nogvim));
+	    emsg(_(e_gui_cannot_be_used_not_enabled_at_compile_time));
 	    return;
 	}
 #endif