Identify and report network failure errors

[aforcier]

Identify ConnectException and UnknownHostExceptions in BaseRequest and report them as a distinct error (not UNKNOWN) - and float them up to OnChanged events (instead of reporting as GENERIC_ERROR).

We can get a ConnectException when the connection is interrupted while a request is being sent (e.g. turning on airplane mode), and we can get an UnknownHostException when attemping to make a request while already in airplane mode or lacking a connection.

It's possible we might get UnknownHostException in other cases as well, so we might not want to identify this one as a NO_CONNECTION error, but rather as a NETWORK_ERROR - but we should think through the various GenericErrorTypes anyway.

Note: This has been done for media uploads, which use OkHttpClient directly and don't share the same error reporting as the other endpoints (#505).

[mzorz]

This one was closed in #505

[aforcier]

@mzorz this issue isn't actually done - we still need to handle these exceptions in the general case - all #505 did was do it for media uploads.

[mzorz]

thanks @aforcier!