Add support for passing back generic warnings to Grafana

[mapno]

Is your feature request related to a problem? Please describe.
As part of #899, in #1007 we're implementing the option to return partial results if block queries fails. Returning partial results needs to be communicated to the frontend, so it can warn the user of the potential missing data.

Describe the solution you'd like
We should be able to pass generic warning to the Grafana frontend for this case and other possible returns.

[joe-elliott]

I would like to see this issue gain some momentum. It would be nice to have a field on every API response like "warning" or "message" that would be displayed by Grafana or whatever was consuming our API.

It occurs to me that we may want to cut a v2 of the api to accomplish this w/o breaking changes.

[github-actions]

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