run_tasks: Do not mask missing task dependencies

run_tasks: Do not mask missing task dependencies

While doing packaging work, I noticed that teuthology began claiming it
couldn't find the 'tests' task. After some slightly painful debugging I
realized the issue was that the task was trying to import pytest, which
wasn't installed. The ModuleNotFoundError that was being raised was
being confused with the exception that would be raised if the task
couldn't be found at all. With this change, we see the root cause.

Signed-off-by: Zack Cerza <zack@redhat.com>