Relates #264.
This PR proposed and implemented a way of defining workflow failure
state:
A workflow job fails if one of the conditions below satisfies.
* At least one node runs into states `canceled` or `error`.
* At least one leaf node runs into states `failed`, but no child node is
spawned to run (no error handler).
Signed-off-by: Aaron Tan <jangsutsr@gmail.com>
* First pass, adapt singleton task manager to process messages and run
jobs based on events instead of a busy loop.
* Still need to make message handing run in celery, not in a consumption
loop
