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.
The first is the log without details (the current log), the second is intended to be the log with additional details (the annotated tree from each phase, say.

Based on our conversation yesterday, we might go a different way here. Maybe instead of a Cord, you get back a list of objects each containing a label and RenderedTree. Some might be identified as "internal", for detailed logging.