Refactor/vows assertion errors

Originally, I wanted to make VowsAssertionErrors expect a single positional argument (the error message) followed by `**kwargs` (which would be used as arguments for `str.format()` for printing, etc.)

That ended up being incredibly difficult.  :P

So, I fell back to `str.format()`’s positional arguments.  So, no more `%s` ugliness.  

I also tweaked a few other small things along the way.  Have a looksee! :D