Fix error message when using invalid VM as a template for DispVM
Fix error message when using invalid VM as a template for DispVM

Don't crash (producing misleading error) when checking if
template_for_dispvms=True.

Fixes QubesOS/qubes-issues#3341

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