Fix bug where neither coinc far is calculated if no external sky map

With the current design the code exits while calculating the coincidence far (will first try space-time and then fall back to temporal because of !567 (merged)) if there is not an external sky map present. Instead we should use a try-except structure that continues to calculating the purely temporal coincidence far instead.

See the related Sentry Error: https://sentry.io/organizations/ligo-caltech/issues/1010794996/?environment=playground&query=is%3Aunresolved&statsPeriod=14d