The text of the startup failure notification is unhelpful due to lack of
space. Touching the notification now launches an activity that gives details
of the problem and what can be done about it.

Closes #38

We don't need two separate executors for long-running IO threads.

Of course, not all devices support Bluetooth...

Hopefully this gives users a better understanding of whether their
messages and contacts are accessible than "Briar is running" and "Quit".

GroupActivity needs to receive a similar treatment.