[amp-story-player] Base url should be use as fallback when cache can't be accessed.

## The player should show the story no matter what

When using `amp-cache` attribute, even if the cached story can't be access for any reason, it should display the non cached story.
It's already the case when the story isn't cached yet, but not after.

## Use cases

We already identify 2 cases where the story doesn't display :

- The story is not compliant. Even when the story is not compliant, the player should display the story from base url (even if we try to make stories always compliant, sometimes something can go wrong).
- Cache is inaccessible. For example, when an adblocker is blocking amp cache its displaying a message, but the story should be shown from base url instead (cf images).


Start by tracing the amp-story-player handling of the amp-cache attribute and reproduce the two reported cases: a non-compliant story and an inaccessible cache. The work is done when the player displays the story from its base URL in both cases, while retaining existing cached-story behavior.

Written by the indexing model from the issue text.