Refine error behavior of bin/search index

Summary: Fixes T5991. If all requested documents failed to index, consider this a catastrophic failure and exit with an error code.

Test Plan:

• Ran bin/search index --type TASK, observed successful exit despite a small number of un-indexable documents.
• Ran bin/search index PHID-TASK-xxx for an invalid task, observed exception on exit after complete failure.
• Ran normal indexing through daemons.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5991

Differential Revision: https://secure.phabricator.com/D14174