if no bridges are usable, tor should report a bootstrap error

In testing Tor Launcher changes while working on #9445 (moved), Kathy Brade and I found that when tor is only configured with bridges it cannot use due to a lack of PT plugins, the bootstrap process just stops making progress. It would be much better if an error was reported via the control port. Here is our torrc:

AvoidDiskWrites 1 bridge obfs2 91.143.91.97:34770 ControlPort 9151 CookieAuthentication 1 DataDirectory /Users/brade/dev/tor/tbb-for-tl-testing/TBB-3.0a2.app/Contents/Resources/Data/Tor DirReqStatistics 0 GeoIPFile ../../Contents/Resources/Data/Tor/geoip Log notice stdout SocksListenAddress 127.0.0.1 SocksPort 9150 UseBridges 1

Here is the tor log:

Sep 04 11:37:33.689 [notice] Tor v0.2.4.14-alpha (git-f5729b8c1d45933f) running on Darwin with Libevent 2.0.21-stable and OpenSSL 1.0.1e. Sep 04 11:37:33.689 [notice] Tor can't help you if you use it wrong! Learn how to be safe at https://www.torproject.org/download/download#warning Sep 04 11:37:33.689 [notice] This version is not a stable Tor release. Expect more bugs than usual. Sep 04 11:37:33.710 [notice] Read configuration file "/Users/brade/dev/tor/tbb-for-tl-testing/TBB-3.0a2.app/Library/Vidalia/torrc". Sep 04 11:37:33.714 [notice] Opening Socks listener on 127.0.0.1:9150 Sep 04 11:37:33.714 [notice] Opening Control listener on 127.0.0.1:9151 Sep 04 11:37:33.000 [notice] New control connection opened. Sep 04 11:37:33.000 [notice] New control connection opened. Sep 04 11:37:34.000 [notice] Bootstrapped 5%: Connecting to directory server. Sep 04 11:37:34.000 [warn] We were supposed to connect to bridge '91.143.91.97:34770' using pluggable transport 'obfs2', but we can't find a pluggable transport proxy supporting 'obfs2'. This can happen if you haven't provided a ClientTransportPlugin line, or if your pluggable transport proxy stopped running.