Add CorruptShareError as a superclass for all corrupt share errors. refs #1566

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Make the cloud backend report corrupted shares correctly. fixes #1566

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>
Cleanup: improve error reporting for DataTooLargeError
(and fix an off-by-one error in MutableDiskShare._write_share_data).

Signed-off-by: Daira Hopwood <david-sarah@jacaranda.org>