Changed (throw) to (throw+) in (default-error-handler).
Added unit tests for (default-error-handler).

The added tests make sure that (default-error-handler) can propagate
both slingshot error maps and normal exceptions.
Made (dire.core/default-error-handler) public.

Adjusted the unit tests so they no longer use Midje's
(testable-privates) function.
Merge pull request #19 from johnworth/develop

Changed (throw) to (throw+) in (default-error-handler)