dispvm: error out on saved-cows.tar creation error
dispvm: error out on saved-cows.tar creation error

If it fails - for example because of too restrictive volatile.img
permissions, subsequent DispVM will not be really disposable.
The original permissions issue should be fixed by previous commit, this
one makes sure that such errors will not be ignored.

Fixes QubesOS/qubes-issues#2200

woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin