When an in-process worker subtask fails permanently, don't fatal the whole process

Summary:
Ref T13552. Fixes T13569. Currently, if a process uses in-process tasks (usually, a debugging/diagnostic workflow) and those tasks (or tasks those tasks queue) fail permanently, the exception escapes to top level and the process exits.

This isn't desirable; catch the exception and fail them locally instead.

Test Plan:
With a failing Asana integration and misconfigured Webhook, ran bin/repository reparse --publish ....

• Before: fatals on each substep.
• After: warnings emitted for failed substep, but process completes.

Maniphest Tasks: T13569, T13552

Differential Revision: https://secure.phabricator.com/D21459