capture exceptions thrown by the MongoDB driver

If a plan is constructed and submitted to the MongoDB driver, and then an error occurs inside the driver, you now can see all the debug logging etc., plus a line with the message from the driver's exception.

Also, added a REPL command to hide the detailed logging: `set debug = 0`. The idea is that level 2 will get you even more verbose logging when #50 is implemented.
Sure, but in this case I'm not sure what's a sensible approach. For now, I'll change it to produce two logs and just pick the right one to log. As you can see, it's not currently implemented anyway. That will be part of #50.