We can log the actual object to the console, while showing user something like "Unknown error has occurred".

CL: https://chromium-review.googlesource.com/c/426840/

CL: https://chromereviews.googleplex.com/562867014/

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra.git/+/7452004ffa2c198f2d379c348970dcb80a643ce6

commit 7452004ffa2c198f2d379c348970dcb80a643ce6
Author: Sergiy Byelozyorov <sergiyb@chromium.org>
Date: Wed Jan 11 12:48:38 2017

Format error object and fix polymer tests

BUG= 672953 
TBR=tandrii

Change-Id: I1b0336fce6e196b0436542fa8cd70db22b6515f9
Reviewed-on: https://chromium-review.googlesource.com/426840
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>

[modify] https://crrev.com/7452004ffa2c198f2d379c348970dcb80a643ce6/go/src/infra/appengine/test-results/frontend/src/test-results-flakiness.html
[modify] https://crrev.com/7452004ffa2c198f2d379c348970dcb80a643ce6/go/src/infra/appengine/test-results/frontend/test/test-results-flakiness.html