Here is a report that has fails for both expect and assert.
(It also has passes).
http://mozmill-sandbox.blargon7.com/#/functional/report/16dc29d7a411d565992bbd7dc84ee26d

I have pushed another commit to properly indent the JSON output. Looks much better now.

Given these rather large error stacks, I'm getting in favor of not limiting in size the tooltip.
When we want to see details, it might be better to have it all displayed in one big box. Otherwise you always need to copy it into a local texteditor. And this is a bit cumbersome. Not sure if a textarea might not be better for this purpose (easy copy/paste)