patch 8.2.3986: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/highlight.c b/src/highlight.c
index d72fa83..253ed5b 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -1053,7 +1053,7 @@
 		color = cterm_normal_ul_color - 1;
 	    else
 	    {
-		emsg(_("E453: UL color unknown"));
+		emsg(_(e_ul_color_unknown));
 		return FALSE;
 	    }
 	}