5283: Improve reliability of crunch-job output collation.

5283: Improve reliability of crunch-job output collation.

* Check the results of all pipe opens, exit statuses, and writes.
  Log any problems.
* Have fetch_block return undef when it encounters trouble, rather
  than dying.  create_output_collection already checks for this, so it
  effectively bubbles up the error.
* Retry all of the associated API calls.
* Kill the manifest creation pipe if we give up on it, per the TODO.

This probably won't resolve #5283, but hopefully these changes will
give us additional information to help diagnose the problem.