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/userfunc.c b/src/userfunc.c
index b7f9dfc..4ba1f3e 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -339,7 +339,7 @@
 	    }
 	    else if (any_default)
 	    {
-		emsg(_("E989: Non-default argument follows default argument"));
+		emsg(_(e_non_default_argument_follows_default_argument));
 		goto err_ret;
 	    }
 
@@ -3850,7 +3850,7 @@
 
 	if (cp != NULL && cp < end)
 	{
-	    semsg(_("E884: Function name cannot contain a colon: %s"), start);
+	    semsg(_(e_function_name_cannot_contain_colon_str), start);
 	    goto theend;
 	}
     }