(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.
Are we sure we want to make response this generic?  Even though the corresponding message would be written to the master log, I still think it's helpful to see something a little more unique about the error go back up to the client.  Has helped me before with coordinating the error as seen from one client with the corresponding error among various activities in the master log (e.g., various clients connecting to the master around the same time that may be causing failures for different reasons).  Maybe we could just write the Exception's message back in the error response?