QA: Browser test for Thank feature in Flow

Adjustment to work by Benjamin Chan

thank.feature tests for two scenarios:
- Anon does not see Thank button
- Cannot thank own post

Bug: 61930
Change-Id: Icd9afc0a12b228bdbb682959e527cdad5d3253b1
Fix error handling

There's a couple of changes here:
* added new findUpward method: use with caution as it can potentially seek a lot
  of DOM nodes (but I'm ok with it - it's limited: html will never be nested
  deep enough for it to cause problems)
* made error handling more generic; no need to pass container - just have error
  container somewhere near ;)
* a bunch of GET calls had no errors displaying (only removeError was called)
* switched a couple of flow_errors in templates (mostly to get them to show in
  processing GET stuff)

Bug: 68021
Change-Id: Id29473cabda78fa9ea5d9ca7d996e9485b0bdbf6
Merge "Fix error handling"
Merge "QA: Browser test for Thank feature in Flow"