Unlock when promoting to exclusive lock fails

In certain cases changeLock to EXCLUSIVE fails
and throws LockedException. This leaves the
file locked as SHARED in file_put_contents,
which prevents retrying (because on second
call file_put_contents takes another SHARED
lock on the same file, and changeLock doesn't
allow more than a single SHARED lock to promote
to EXCLUSIVE).

To avoid this case, we catch the LockedException
and unlock before re-throwing.

Signed-off-by: Ashod Nakashian <ashod.nakashian@collabora.co.uk>