test_runner: print failed coverage reports with dot runner

What is the problem this feature will solve?

When running tests with both the dot reporter and coverage reports, if the coverage report fails there is no output (other than the dots) but the program exits with a failure status code.

What is the feature you are proposing to solve the problem?

Similar to this PR, it would be nice to display some sort of failure reason so its clear why the command failed.

What alternatives have you considered?

No response