Due to an issue with the interaction between mocha and karma-mocha [1],
uncaught exceptions or promise rejections that occur in between tests
can cause the test run to stop immediately yet incorrectly report that
all tests executed succesfully and hence CI runs / Jenkins builds
incorrectly report their status.

This commit works around the issue until the problem is resolved in
mocha / karma-mocha by adding an `afterEach` hook which will re-throw
any uncaught exceptions and ensure that mocha notices them and reports
the failure correctly to Karma.

[1] See comments on https://github.com/hypothesis/client/issues/2249