dispvm: show error in tray when DispVM startup failed
dispvm: show error in tray when DispVM startup failed

Fixes QubesOS/qubes-issues#1457

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