job_coordinator: fix the return value of retry_task in case of failure

This will fix the error messages when there are many task failures. Now the
erorr message clearly shows that the reason the job failed is because a single
task had many failures.
Merge pull request #572 from pooya/571

job_coordinator: fix the return value of retry_task in case of failure
disco core: avoid creating an instance of worker just for reading blobs
disco core: remove the Params class from disco core

This has been deprecated for a while. This might break some of the clients that
should switch to using the new import location
disco worker: rename ClassicFile to StreamCombiner

The old name did not make much sense.
disco core: remove the classic_iterator alias for result_iterator

The name result_iterator is by no means accurate, but it is widely used by the
the disco jobs. In the future, it should be renamed to something like
disco_iterator.