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/syntax.c b/src/syntax.c
index 60c35bd..fea317a 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -3723,7 +3723,7 @@
 		id = syn_namen2id(arg, (int)(arg_end - arg));
 		if (id == 0)
 		{
-		    semsg(_(e_nogroup), arg);
+		    semsg(_(e_no_such_highlight_group_name_str), arg);
 		    break;
 		}
 		else
@@ -3921,7 +3921,7 @@
 	    {
 		id = syn_namen2id(arg, (int)(arg_end - arg));
 		if (id == 0)
-		    semsg(_(e_nogroup), arg);
+		    semsg(_(e_no_such_highlight_group_name_str), arg);
 		else
 		    syn_list_one(id, syncing, TRUE);
 	    }