#35 gracefully handle exceedance of api.github.com query limitation

I run a homelab with several applications that regulary query api.github.com. Thus, resulting in an exedance of the query rate. Using a network sniffer, I figured out that freeShop (i tried 2.2.0 and -unstable) does query api.github.com during cache update as well. In my situtation the cache update was unsuccessfull because of the exceeded limit and freeShop bailed out, froze or threw an error. Since i have a dynamic internet ip a reconnect results in a new ip, after each reconnect I am able to update the cache exeactly once, before my other applications exceed the query limit. Please add an error code/message for gracefull handling of the scenario: 1) if the cache is not downloaded at all: - other applications in your network are responsible for exceeding the api.github.com query rate. Please deactivate those and/or reconnect your internet connection. (you could even add the date/time when the exceedance expires, it is provided in the ip header in unixtime) 2) if the cache is present: - same message from 1, but as warning, not as error. A user sould be able to continue to use freeShop