Try to rollback an invalid partial mount/unmount

[kolayne]

In the current implementation, it is possible that during a mount/unmount operation, each comprising two system call (mount/unmount the internal overlayfs + create/delete the corresponding activemount file), if the second attempted system call fails, we end up in an invalid state with the volume being partially mounted/unmounted.

In such a situation, let's attempt to rollback. E.g., if we mounted a volume first, then tried to create a file but failed, attempt to unmount the volume before reporting the error. When possible, this will restore the volume's clean state.