pxar: use anyhow::Error in PxarBackupStream

pxar: use anyhow::Error in PxarBackupStream

Instead of storing the error as a string in the PxarBackupStream, we
store it as an anyhow::Error. As we can't clone an anyhow::Error, we take
it out from the mutex and return it. This won't change anything as
the consumation of the stream will stop if it gets a Some(Err(..)).

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>