phab sink: Gracefully handle non-existent or inaccessible tasks

lib/phab.py:
Client.lookupPhid():
Instead of raising an exception if the task cannot be found, return None
Client.editTask():
Return a boolean indicating whether or not editing happened.
In particular, return False if the task is not found.

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Related MRs:

• !25 (merged) (This MR)
• !24 (merged)
• main

Depends-On: !24 (merged)