loading invalid configuration files should throw descriptive messages

per [this conversation](https://github.com/nature/pa11y/commit/e08fc3d80cc73c221847570a88aab824a6df81e3#commitcomment-3655399), i'm updating the error message to be more descriptive and easier to debug
Return with error count as exit status.
Changes to feature tests to cater for non-zero error codes
Fix the way exec options are set in feature tests (random fails)
Simplify the load-config tests