Added useful error messages to the labels plugin
Added useful error messages to the labels plugin

Summary:
This changeset uses Chrome's Accessibility Developer Tools in place of my
original label-detection logic.

I used this information to provide useful error messages.

I also made the errors tab items expand/collapse. The first is expanded by
default and the rest are collapsed.

Test Plan:
. `webpack-dev-server --hot`
. `localhost:8080/webpack-dev-server`
. Activate the "Labels" plugin
. See two errors in the errors tab
. See the first is expanded and uses the "email" ID from the violation input
. See the second is collapsed, but can be open with a mouse click
. See the second error asks the user to make their own ID

Reviewers: michelle

Differential Revision: https://phabricator.khanacademy.org/D18211

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