Report web-test-runner test failures

[flavorjones]

The test runner reporter now emits failure details, taking care to sourcemap the stack trace.

Example:

FAIL: HTMLSanitizer > failure test
  Message: Assertion Error
  Expected: 2
  Actual: 1
  Stack:
        at Object.<anonymous> (src/test/unit/html_sanitizer_test.js:32:11)
        at runTest (http://0.0.0.0:8000/qunit/qunit/qunit.js:2983:35)
        at Test.run (http://0.0.0.0:8000/qunit/qunit/qunit.js:2966:9)
        at http://0.0.0.0:8000/qunit/qunit/qunit.js:3259:16
        at processTaskQueue (http://0.0.0.0:8000/qunit/qunit/qunit.js:2525:26)
        at advanceTaskQueue (http://0.0.0.0:8000/qunit/qunit/qunit.js:2508:5)