[FileAPI] FileReader.abort() and File[Saver|Writer].abort have different behaviors

The abort behaviors of FileReader and File[Saver|Writer] differ.  The
writing objects throw if the abort method is called when a write is not
currently under way, while the reading object does not throw.

The behaviors should be consistent.  I don't particularly care either way,
but I believe Jonas would like to change FileReader to match
File[Saver|Writer].

- Kyle