storageproxy: Report fsync failures with a distinct error code

Fsync failures are special because they may indicate a failure of an
operation before the current operation. Report these cases as a new,
distinct error.

Test: Cause fsync failure and check error response
Bug: 239105007
Change-Id: Ie9d4a1949586e90006256c975786e21ced655e66
Merged-In: Ie9d4a1949586e90006256c975786e21ced655e66
(cherry picked from commit 1c75d1e3)