Ditch `%s` string formatting in assertions
Ditch `%s` string formatting in `VowsAssertionError` class
Tweaked `VowsAssertionError` class

* added a few sanity checks
* instance variable `msg` is now `raw_msg` (suggesting that it is intended for use with `str.format()`)
Made `Vows.create_assertions` friendly to `str.format()`
Minor improvements to `VowsReporter.print_traceback()` (in `reporting.common`)

* every time I read “exc_blah”, I think “**execute** blah” instead of “**exception** blah”.  So I renamed these to “err_blah”. :-P
* Also renamed `err_value` to `err_obj`, since it more accurately describes what the variable contains
* Also, this method was manually indenting its output; changed to use the `indent_msg()` method
Merge pull request #71 from Zearin/refactor/VowsAssertionErrors

Refactored VowsAssertionErrors