Error message with link to new harness when using old harness.

- Added the ability to parse urls in error dialogs (sanitizes all other text).
- Detect old harness via 404 error and show custom dialog.
  - Added link to [latest harness releases](https://github.com/MobileChromeApps/chrome-app-harness/releases) in error.

Fixes #2418 
Review @devoncarew 
Can we be sure that a 404 (or higher error) is caused by the user having an out-of-date app harness?