Provide different error message for DAS lookup issues.

Fix the longstanding bug where a das failure is indicated as an error in
step two. This introduces a new DAS_ERROR exit code which is correctly
associated to step1. This is still not enough to catch the cases in
which is the lookup for pileup which fails, but at least is a good
start.
Merge pull request #6641 from ktf/proper-das-error

Provide different error message for DAS lookup issues.