corrupted introducer.furl causes subtle startup breakage

[warner]

Timothy Fitz discovered last night at the hackfest that a corrupt BASEDIR/introducer.furl (caused by a broken link on our TestGrid page) causes the node to throw an exception partway through the startup process, leaving it in a funny state. Basically the web server got attached, but the client instance didn't get attached (which happens in startService). This made the welcome page display a traceback complaining about being unable to adapt the nevow context to the IClient interface.

We had some code in place (at rob's prompting) to detect exceptions during startup and then do sys.exit(). I suspect that there's an error path that bypasses this. Note that the exception *did* show up in twistd.log, but the node ran anyways.

The #71 "node probably started" feedback should cover problems like this: if node startup fails in this way, the 'tahoe start' command should *not* display success.