Event 'error-opening' emitted before listener attached

Hi there,

I was setting this up as per the 'Connecting to MongoDB' section of the 'Getting Started' instructions in the documentation, and found that a poorly formed url would cause the system to exit instead of generating the expected promise rejection. From the code it looks like the url is tested in the monk constructor and an 'error-opening' event is emitted before an event listener is attached in the 'then' function.

This all means that the error is not caught and goes unreported, and the program fizzles out without any indication of the error that has just occurred, in my case it was 'Error: slash in host identifier'.

Start with the Connecting to MongoDB section of the Getting Started documentation and reproduce the behavior with a poorly formed URL. Inspect the monk constructor and the then function, where the issue reports that the URL error is emitted before the listener is attached; done means the malformed URL produces the expected promise rejection instead of an unreported process exit.

Written by the indexing model from the issue text.