Error is misleading when a value isn't found for a runner

[chadfawcett]

In #5 the error eluded to the runners input needing to be a string of comma separated runner ids even though that was properly provided.

The actual issue was that there was no env value set for the runner id. It appears the error handling for the comma separated string doesn't accept hyphens (-) in the runner ID, even though this seems to work with the program, so it outputs the wrong error message.

[RyanHecht]

Reach: Low -- custom autograding must be in use, 0

Impact: High -- misleading error message, unclear how the user unblocks themselves, 2

Confidence: High, we know what the problem: 1

Effort: Low, 1

priority: 1