obscache: update the variable tracking on disk state after write
Since we updated the on disk content, we should update that value.
In practice the object will likely be discarded after the write, but there is
nothing wrong in being more correct.
obscache: ignore permission and OS errors when writing
In practice we always update this cache along side a new transaction, with lock
taken so permission issues should not happens. However, staying on the safe side
is better. Errors during Cache update should not block Mercurial operations.