clarify reset label colors function

prefs.js

@@ -1398,10 +1398,10 @@ function labelColorReset() {
 	try {
 		var labels = getSelectedLabels();
 
-		var ok = confirm(__("Reset label colors to default?"));
+		if (labels.length > 0) {
+			var ok = confirm(__("Reset selected labels to default colors?"));
 
 			if (ok) {
-
 				var query = "?op=pref-labels&subop=color-reset&ids="+
 					param_escape(labels.toString());
 
@@ -1412,6 +1412,10 @@ function labelColorReset() {
 					} });
 			}
 
+		} else {
+			alert(__("No labels are selected."));
+		}
+
 	} catch (e) {
 		exception_error("labelColorReset", e);
 	}