On master, we changed the Store::update() function to do nothing more
than calling Store::_update() and wrapping an error.
On the libimagstore/store-tests branch, we modified the
Store::_update() function to contain an additional flag in the
parameters, whether the File-Lock-Entry internal `precense`-value should
be updated.
This merge commit resolves that conflict.
We must return an error if the file is not found here.
This is because if we unwrap() here, we panic if the store action was
Store::retrieve() and there wasn't a file there. We then unwrap() on
None and panic because of this, causing all other tests to panic as
well because the Mutex gets corrupted.
The store handles FileNotFound errors on its own, so it is safe to
return the error here.
