Task doesn't handle bad participant lists gracefully

[mfeit-internet2]

There was a problem in the netreach participants method:

$ pscheduler task netreach --network 127.0.0.1/32
Submitting task...
Traceback (most recent call last):
  File "/usr/libexec/pscheduler/commands/task", line 681, in <module>
    lead = participants["participants"][0]
KeyError: 'participants'

Debig shows that the participants method returned an error:

2026-05-26T20:50:18 Got participants: {'valid': False, 'error': 'Network must have at least two host addresses.'}