Handle proxy errors in connection assist

After forgetting to clear invalid proxy settings, I ended up in a bootstrap failure.

So, I arrived to the connection assist screen to choose a location, however I noticed this message:

Connecting to proxy failed (CONNECTREFUSED).

When we encounter this kind of error, we should send the user to the advanced settings, rather than trying connection assist, esp. because Moat should (theoretically) observe these proxy settings as well, so trying a bridge should also fail.

This case is closer to an offline state, rather than the need of a bridge.