the Box is not reachable (once the set up is done) from a different network

[michielbdejong]

When connecting to a Box for the first time, the app should note and remember the tunnel URL, and automatically switch to that if the Box's local hostname is not reachable.

As noted in #64 (comment).

[michielbdejong]

Also, if the Box does not have a tunnel, or the user tries to do an action that can only be done via the local network, and error message should be displayed, saying 'please retry this at home'.