Fixed retryable tasks for raven.py; Fixes #84

This merge request is to make much of the raven.py workflow retryable and in canvas structure is applicable.

The following sentry errors should be addressed from !581 (merged):

Removed call creating this error: https://sentry.io/organizations/ligo-caltech/issues/1164177171/?query=is%3Aunresolved

Block empty search results from being processed: https://sentry.io/organizations/ligo-caltech/issues/1164176937/?environment=playground&environment=production&query=is%3Aunresolved

calculate_coincidence_far should be executed now as well.