remove UnhandledErrorDialog

Summary:
the dialog was actually broken through at least all of plasma5 :/

this is no longer necessary. it was used for when random unexpected HTML
would get returned. there is no longer unexpected html:

- the api always returns json blobs
- should the api crap out we'll get protocol errors (HTTP 500 or some such)
  which is also forced into a serialized form and raised as a
  ProtocolException to be handled like errors from the API directly

we always get one "standardized" error message and never an
extended html to display.

Test Plan: builds

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22393