Streamline :next-checker predicates

Specify the maximum permissible error level instead of mucking about with custom predicate symbols.

# TODO

- [ ] Update [flycheck-hdevtools](https://github.com/flycheck/flycheck-hdevtools/blob/master/flycheck-hdevtools.el)
- [ ] Update [flycheck-d-unittest](https://github.com/flycheck/flycheck-d-unittest)