A common superclass for ZoomExceptions.

Defines a StorageException, which is a generic Exception class to be a
common super class for various Storage / Zoom related exceptions.
StorageException is part of the public Storage API.

MN-2999

Change-Id: I47b5da994f30d84361581842a0e9258ffcf3832a
Merge "A common superclass for ZoomExceptions."