[Reader] Improve Tag Preview error / empty screen

This is an enhancement suggestion for the Reader page when it tries to fetch a tag but that tag doesn't have any posts.

Currently, in that situation, an error / empty screen is shown, but that screen does not tell the user which tag was being searched, which looks odd since the error message could be "No posts with this topic" (which topic though?!).

The proposal is to show the same header we show for successful results even for errors, so the user knows what content the app tried to fetch. Example in the screenshots below.

Current Success Screen	Current Error Screen	Proposed Error Screen

Expected behavior

The app shows which tag we could not fetch, in case of error / no posts found.

Actual behavior

An error referencing "this topic" but no reference of the tag / topic.

Steps to reproduce the behavior

Currently, the easiest way of falling into that scenario is by doing the test steps in #17800, since that PR implements a button that redirects to the Reader for viewing tags that are not being used currently (will be in the future).

Tested on Samsung Galaxy S21+, Android 13, WPAndroid and JPAndroid 21.5

Start with the Reader tag-preview flow described in this issue and review the test steps in #17800 to reproduce the empty or error state. Compare the current and proposed screenshots; done means the error or no-posts screen identifies the tag being fetched while retaining the existing successful-results header.