[OAuth2] - Error handling on authentication errors

[SamuAlfageme]

As pointed out in #5813 there's some more error handling logic to be included on the auth. workflow once it's ready on the app/core; e.g. depends on:

• Clicking "Cancel" on the authorize page does not notify the client. oauth2#46
• Entering the wrong password once break the authentication flow oauth2#47 Login screen: entering the wrong password breaks the redirect core#28129
• Make OAuth work out of the box with Apache servers (install dependencies, etc.) oauth2#49 (this was the original cause of Authentication with OAuth2 #5668 (comment))

(Migrating the tags from #5813)

[ogoffart]

I believe most of the issues should not be fixed.
Please re-open tasks for specific issues.