(SERVER-88) catch errors before they get to Jetty

Add a 'catch-all' middleware to both the Master and CA ring handlers
which catches any Exception, logs it, and returns a simple error message
in the response body.  This prevents Exceptions from propagating all the
way out to Jetty - if this happens, Jetty will generate HTML from the
stacktrace and return this to the client.
Hmm ... not sure I agree with you here.  This has the advantage of being a bit simpler in that you can simply crank the log level up to DEBUG to see this logging, whereas jetty access log takes a bit more configuration.  Also, this logging is useable outside of jetty (say, interacting with the ring handler directly in tests or in the REPL).  Not really a big deal ... but seems worth leaving along, at least for now.