qemu: monitor: Provide more information in generic block job error

qemu: monitor: Provide more information in generic block job error

The qemuMonitorJSONBlockJob handles a few errors internally. If qemu
returns a different error we would report a rather unhelpful message:

 $ virsh blockpull gluster-job vda --base /dev/null
 error: internal error: Unexpected error

As the actual message from qemu contains a bit more info, let's use it
to report something a little more useful:

 $ virsh blockpull gluster-job vda --base /dev/null
 error: internal error: Unexpected error: (GenericError) 'Base '/dev/null' not found'