fix(datepicker): fix error message

- Change error message to reflect what is officially supported as a model value

Closes #5198
Fixes #5191

Author: wesleycho

src/datepicker/datepicker.js

@@ -119,7 +119,7 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
       if (isValid) {
         this.activeDate = dateParser.fromTimezone(date, ngModelOptions.timezone);
       } else if (!$datepickerSuppressError) {
-        $log.error('Datepicker directive: "ng-model" value must be a Date object, a number of milliseconds since 01.01.1970 or a string representing an RFC2822 or ISO 8601 date.');
+        $log.error('Datepicker directive: "ng-model" value must be a Date object');
       }
     }
     this.refreshView();