The frontend UI assumes transitions are always available and crashes if
none are present. Resolving the crashes resulted in a completely broken
UI, so it's safe to assume that transitions are a core requirement for a
working OBS.
Crash reports indicate that obs-transitions is failing to load for some
users, so we should handle the case where no transitions load and abort
with an error instead of crashing.
Richard Stanway
e86c7e501e