Allow to work with cache on requests errors, common func to get date

Allow to work with cache on requests errors, common func to get date

+ Create common function to get the current date and time.
+ Identify request errors, specifically after trying to fetch data via http/rsync without success. This helps to identify if a whole repository can't be downloaded after a considerable time (it can be configured).
+ Allow to work with local files even when there was a download error.
+ Add 'stale-repository-period' argument to set the time period that must lapse to warn about stale repositories (this will be logged to the operation log).