backups: report exception on backup write error
backups: report exception on backup write error

This is only partial solution - this happens in separate thread so main
thread may not notice this problem (and simply will wait on work queue).

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