[MERGE] versionedfile.py code cleanups

[Ian Clatworthy]

More minor improvements. A trivial question as well for the style gods ...

We commonly use the pattern of having a method foo in a base class that
does the common stuff (e.g. parameter validation) and then calls to
method _foo to do the real work. In the base class, _foo simply does this:

  raise errors.NotImplementedError(self.foo)

Derived classes are responsible for implementing _foo. Given that,
should the NotImplementedError take self.foo or self._foo as it's parameter?

Ian C.
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