Inform user if reachable bridges drop below a configurable fraction/number
It would be very useful for a number of reasons if Tor Launcher could pop up some kind of alert if some fraction of their bridges become unusable (ie when less than 50% are reachable, or perhaps less than min(33%,1)).
In that case, it should instruct the user to obtain more bridges, and give them instructions and/or a bridgedb link specific to their PT type (ie https://bridges.torproject.org/bridges?transport=obfs3).
The primary motivation for altering the user is that if bridges either go down or get blocked, it will be far easier for the user to obtain more if the user still has at least one working bridge to work with (to connect to gmail or visit a link).
One way to do this might be to use the control port command 'GETINFO entry-guards" on a timer, but an event-based approach using the "GUARD" event could also work (but would require substantially more bookkeeping and may be error prone in the face of GUARD event state transition oddities).
Tor Launcher probably should also not issue the warning if all bridges become unreachable at once, and/or if Tor detects a cease in network activity (which does not seem to be directly exported to the control port at this point :/). We don't want to alert the user every time they walk out of range from a wifi hotspot or similar situation.
This option also should not be active if the user is not using bridges.