Buffering: improve buffering indicator and error states

The app's UI currently assumes that once you start a stream, the stream will commence fairly soon. Since we don't control a customer's connection and don't host the podcast content this isn't guaranteed. We need to add some user interface indicators of buffering progress and more information about any errors or slow downs around that. This issue will require some design input.

The issue names no files, tests, or entry points. Start by clarifying the required buffering states, error details, and design direction for the iOS streaming UI; done means the agreed indicators and error information are implemented and verified for slow or failed stream starts.

Written by the indexing model from the issue text.