Update mongokit pattern

There were some code issues with the current version of the mongokit docs. Specifically, with format strings when you want to format a part of the string and then have the value placed in, you need to use format immediately to avoid a TypeError when mongokit evaluates the string. Also, uses the mongokit ValidationError instead of a plain old Exception
This paragraph now has some extreme mental leaps, I think your note is better presented as an inline comment.