Move the mount checking into DiskFile constructor

Move the mount checking into the DiskFile object constructor. The
REPLICATE method does not use the DiskFile object currently, so that
method still has to perform its own mount check.

We also refactored the DiskFile exceptions a bit for a hierarchy.

Change-Id: Idcbb5a912d8c7d956dac96ff523c1cf1163c98a6
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Merge "Move the mount checking into DiskFile constructor"