patch 8.2.3970: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more errors to errors.h.
diff --git a/src/gui_x11.c b/src/gui_x11.c
index 07699d3..6ef3474 100644
--- a/src/gui_x11.c
+++ b/src/gui_x11.c
@@ -3229,7 +3229,7 @@
 	    //     gui.sign_width = sign->width + 8;
 	}
 	else
-	    emsg(_(e_signdata));
+	    emsg(_(e_couldnt_read_in_sign_data));
     }
 
     return (void *)sign;