Save to local temporary storage in case of failures

[glpatcern]

In case the remote storage is unreachable or the WOPI session has expired, we may end up with a valid document that cannot be saved.

We should at least save it to some temporary local storage, in order to help with later recovery by operations - or ideally have a mechanism to renew the WOPI access token.

In the code, this corresponds to two comments marked as:
TODO here we should save the file on a local storage to help later recovery!