FR: centralized, consistent UI messaging for plugin error states

This feature request is for making a centralized mechanism to handle error states for TensorBoard plugins, in particular the common cases where they don't have any data to show, or were not properly enabled.

Right now every plugin is responsible for its own "no data" error messaging, which leads to duplicated code and/or inconsistencies, and makes it hard to swap out the messaging in a clean way (e.g. if we wanted different messages for TensorBoard.dev), especially since it's all the way down inside the polymer dashboard components.

Instead, we could imagine handling much of this in the Angular shell, where it's easier to customize. We already fetch /data/plugins_listing to determine which plugins are "active" in the top toolbar, so we could imagine extending this to also distinguish whether an inactive plugin should be rendered at all, or whether it should show an error / help message instead.

We might also consider some other issues with /data/plugins_listing behavior as part of this, e.g. #1856 and #2102/#2133.

Start with /data/plugins_listing and its documentation in http_api.md, then trace how the Angular shell and Polymer dashboard components currently handle plugin activity and error messaging. Done means a centralized, consistent approach for no-data and improperly enabled plugin states, including the inactive-plugin behavior described in the issue.