astroquery cache should fall back to expired cache if remote fails

[aripollak]

Some API backends (like MPC) have extended downtimes, and there is currently not easy way to tell astroquery (post-0.4.6 / #1634) to fall back to an existing expired cached response if the request raises an exception, while still respecting the regular cache expiration for successful requests.
In requests-cache, the equivalent setting would be stale_if_error=True. Maybe switching to requests-cache would be a good improvement, since it would allow deep customization of caching behavior, like an offline mode with only_if_cached=True.

[bsipocz]

Adding request-cache as a dependency would come with some risks (it looks to be a single person project), but it's currently active and seems to have a sizeable downstream user group.
However, I don't think making the switch gets high enough on the maintainers' todo any time soon, so contributions towards it from the most affected modules would be desired.