xC: describe general.editor parse errors

xC.c

@@ -13887,22 +13887,27 @@ build_editor_command (struct app_context *ctx, const char *filename)
 		{
 		case 'F':
 			str_append (&argument, filename);
-			break;
+			continue;
 		case 'L':
 			str_append_printf (&argument, "%zu", line_one_based);
-			break;
+			continue;
 		case 'C':
 			str_append_printf (&argument, "%zu", column + 1);
-			break;
+			continue;
 		case 'B':
 			str_append_printf (&argument, "%d",  cursor + 1);
-			break;
+			continue;
 		case '%':
 		case ' ':
 			str_append_c (&argument, *editor);
-			break;
-		default:
-			print_warning ("unknown substitution variable");
+			continue;
+		}
+
+		const char *p = editor;
+		if (soft_assert (utf8_decode (&p, strlen (p)) > 0))
+		{
+			log_global_error (ctx, "Unknown substitution variable: %#&s",
+				xstrndup (editor, p - editor));
 		}
 	}
 	if (argument.len)