Don't exit early when path doesn't exist

Summary

As a sysop investigating inspection issues, I want houndigrade to report in its results when the device path cannot be found so that I can better diagnose that problem.

Currently, the device paths are type-checked as input arguments. If the device path is not found, input validation fails, the main code is never called, and nothing is written to the results queue. This has a later side effect in cloudigrade of never scaling down the cluster because cloudigrade (currently) needs to find something in the results even if it's just an error.

Acceptance Criteria

• Verify that running houndigrade with an invalid inspection target results in an appropriate error for that target on the results message queue

Assumptions and Questions