Prevent on_cancellation_job & on_completion_job deserialization failure blocking cleanup

[will89]

This addresses https://github.com/salsify/delayed_job_groups_plugin/issues/22. This skips enqueueing the on_cancellation_job or on_completion_job if there is an error deserializing it. Adds an optional proc, error_reporter, that accepts one argument (the raised error) when the job group encounters a deserialization error.

prime: @jturkel