Rework the export failure handling

Currently, the way to signal export failures is by the return value.
This means that if a client doesn't check the values (e.g. burnin), any
failure is being ignore. And this is what we've been doing forever in
burning (not actually testing that the export is successful).

This patch changes the behaviour of ExportInstance: it will abort with
an exception for any error, and removes the custom handling from
gnt-backup. This makes the behaviour consistent for any client (e.g.
RAPI), and it prevents false positives. If, for a given instance, a
subset of disks should not be backed up, the OS scripts should handle
that case.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>