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.
Yes, for sure, I'd say we want to go a different way. Since the 2nd one isn't used now, let's just delete it since we're going to change the approach in the near future.