fs-back: better error handling in fs-backend

fs-back: better error handling in fs-backend

Currently most of the error checking in fs-backend is done by the use
of asserts that would terminate the daemon in case of a single error
on a single request.  This patch replaces the asserts with debugging
messages and terminates the connection on which the error occurred.
With this patch applied I was able to complete successfully over 1000
live migrations with stubdoms.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>