[Extensions] Make chrome://extensions use developerPrivate for error calls
Make the chrome://extensions page use chrome.developerPrivate API for
extension error-related functionality.
Also add tests for the api function, and convert it to a
UIThreadExtensionFunction.
Additionally, fix a bug in extensions.js where calling "showOverlay" on an
already-visible hid the overlay (temporarily).
BUG=461039
Review URL: https://codereview.chromium.org/
973303002
Cr-Commit-Position: refs/heads/master@{#319661}