distinguish between network failure and missing tags

[pravi]

I was using mobile internet and internet connection failed during npm2deb run. npm2deb thought tags are missing and created watch file with fakeupstream. It should have thrown an error instead. I'm not sure if uscan gives different error codes for connection failure and no tarball, we may have to ask uscan developers if there is no such option.

Even after creating watch file with fakeupstream, the network was still down, but npm2deb did not print any error message and upstream tarball was not downloaded.