Settings: give better hint when checking BT "Internet access" fails

When attempting to use a Bluetooth device as an internet source, the
host device might have the setting disabled. If the connection fails
(within a 5 second timeout), display a helpful message letting the user
know what might be wrong instead of silently failing.

Change-Id: If1a8583b04c2b682dd5a4bf5fa7e1b538d0c1488
Signed-off-by: Roman Birg <roman@cyngn.com>