message: Changed error message tested
  The error comes when the link's built instead of when it's parsed. This is a change but I don't think it matters? Have to reject empty array items. Arguably could reject blank crumblessUrl earlier - when state context built? But it's v obscure so not worth it
diff: @@ -4625,7 +4625,7 @@ describe('Navigation', function () {
  ]);
  stateNavigator.navigate('s0');
  stateNavigator.navigate('s1');
  - assert.throws(() => stateNavigator.navigate('s2'), /is not a valid crumb/);
  + assert.throws(() => stateNavigator.navigate('s2'), /Invalid navigation data/);
  });
  });
  