Improve error messages

[aaronbrethorst]

From @alanb2718:

Today's fire drill with the Sound Transit server changes shows that users are confused by the error message they see when the server is down. Currently it says "Error: The data couldn't be read because it is missing." Better would be something more informative and that diagnoses the problem if possible.

If the app can detect the current state, I'd suggest several different error messages: Error: OneBusAway is unable to connect to the internet (show this if there is no connection whatever) Error: the current settings don't allow OneBusAway to access cellular data, so the app isn't able to show any transit information at this time. To fix, either allow OneBusAway to access cellular data, or connect via a WiFi network. Error: Uh-oh. It looks like the server for [region] is down, so OneBusAway isn't able to show any transit information for [region]. Once the server is back up, the OneBusAway app should automatically start working again.

[ualch9]

Additionally, percolate up decoding errors to a Debug screen so agencies can troubleshoot invalid API responses without attaching a debugger.