chore(doc-gen): add formatted error messages to error pages

[petebacondarwin]

This got missed in the doc migration: When there is an error in an
Angular app, extra information is placed in the URL, which can be used
by the docs application to display a more useful message.

This fix adds that back in. The error message templates are extracted
by the minerr tool during build and put into the errors.json file. The
errors-doc processor will load this up and attach these message templates
to the error docs.

The display of these templates was already in place, via the errorDisplay
directive in docs/app/js/errors.js.

(Also, moved the error.template.html file into the angular.js repository
from the dgeni-packages repository as this is specific to the angular.js
project and all the other error related stuff is in here.

[mary-poppins]

Thanks for the PR! Please check the items below to help us merge this faster. See the contributing docs for more information.

• Uses the issue template (#6363)

If you need to make changes to your pull request, you can update the commit with git commit --amend.
Then, update the pull request with git push -f.

Thanks again for your help!