Improve error handling if no internet connection is available
Currently the error message "This seems not to be a bitmask provider" appears if the user wants to add a new provider and there's no internet connection. The error handling in the process of configuring a provider should be improved so that missing internet connection (e.g. no wlan, no sim card or firewall...) is detected as such directly at the beginning. The resulting error message should be more clear, e.g. "The server is unreachable, please check your internet connectivity."