Add response cache for failed requests

[coderReview]

Experimental feature.

Allow cached response to be shown to the dashboard if there is a failure to acquire new data.

Last value is duplicated to the current dashboard end time.

[complacentsee]

I had thought about something similar when looking at the streaming side. Is your thought if the query parameters are identical to attempt the query, and if it fails, just provide the cached value with the current end time?