Bug fix for cancel():
Bug fix for cancel():

1) Jobs with dependencies are not allowed to be canceled. Unfortunately, qless was
    throwing a less-than-helpful error in place of a more meaningful one. That has
    been rectified with this commit.
2) When a job is canceled, if it had previously been dependent on jobs, it should
    remove itself from the lists of dependents of each of its dependencies.

seomoz/qless-core,seomoz/qless-core,backupify/qless-core