Code review comment for charm-hw-health:unknown-on-tool-error

The change makes sense, I agree UNKNOWN is a more appropriate state, and will alert where WARNING will not.

Let's not provide the location of the failed command output, unless we're able to show that output somehow without needing ssh to the unit. Nagios check output needs to be very brief and a single line, and the location of the file doesn't change according to the result of the test.