Improve storage validation after configuration structure update

Besides improving the error message to specify what exactly you
need to do to solve the error, we now don't skip all storage
validations on the test environment, so that you also get a nice error
message if you're running tests. Now if conditions are met to skip
valitaions (test env or env variable) we still make sure the settings
_look_ sane, we just skip verifying the paths exists and meet the given
conditions.