UploadWizard should handle network connection problems better.

Steps to reproduce:

1. Visit https://commons.wikimedia.org/wiki/Special:UploadWizard
2. Turn off your network connection [or check "Offline" in browser dev tools]
3. Try to upload a file

Expected: an error saying that you're not connected.

Actual: an error saying "The server responded with an invalid JSON document. This could be a problem with the API, or you could be using a proxy server that prevents you from uploading files.".

Reported at https://commons.wikimedia.org/wiki/Commons:Help_desk/Archive/2016/10#Upload_error_.27The_server_responded_with_an_invalid_JSON_document.27