API /runs should return 404 instead of empty list

[mfeit-internet2]

The /pscheduler/tasks/xxx/runs endpoint returns an empty list rather than the 404 it should if the task doesn't exist:

$ cpr https://ps01-l.farm.particle.cz/pscheduler/tasks/e24f7abc-21d2-4cb3-88dc-67d30cc38989/runs/74f58e57-178b-4112-9485-13dff578d041
Server returned 404:
Resource not found.
$ cpr https://ps01-l.farm.particle.cz/pscheduler/tasks/e24f7abc-21d2-4cb3-88dc-67d30cc38989/runs
[]
$ cpr https://ps01-l.farm.particle.cz/pscheduler/tasks/e24f7abc-21d2-4cb3-88dc-67d30cc38989
Server returned 404:
Resource not found.

Code is in `pscheduler-server/pscheduler-server/api-server/pschedulerapiserver/runs.py`