A chord is a Group and a task that executes when the Group is done with the results of the group.
If a task fails in that group - nothing gets called at the end of the group execution.

Proposal: NewChordWithError which would make a Chord(Group, SuccessTask, FailureTask) using the following logic:

if all tasks in group succeed:
     SuccessTask(results of group)
else:
     FailureTask(errors of failed tasks)

The FailureTask would take an aggregate of errors of all the failed tasks e.g. func FailureTask(errs []string) error
Any successful completions of the tasks in the group would be 'dropped'

Changes in this PR

• tasks.NewChordWithError() function that takes an additional Signature
• tasks.NewChord maintains backwards compatiblity
• results.AsyncChordResult behavior modified to fetch ErrorCallback results if one of the tasks fails
• worker.go modifications for processing ErrorChord
• Consolidated Chord Callback logic handling in single function: processChord
• Interfaces and Mocks for Server and Worker
• Unit tests
• Integration tests

Note: Unit tests made this a larger change as they required making interfaces out of Server and Worker and injecting those into processing functions. I'd love to take this effort further - but I wanted to keep this change as small as possible. It's already 16 files so I somewhat failed at that.

Let me know if you have thoughts on the utility of the change, etc