use custom error to capture source

[scottlepp]

Currently a lot of plugins, including in core grafana, partner, enterprise etc don't consistently return errors. It's a bit unclear as to when an error should be returned vs returning a response with an error.

If a plugin fails to connect during QueryData, should it return an error? or return a response with an error? Because it is unclear, many plugins return an error in that scenario.

Regardless, rather than change all the plugins, we can just return an error that includes the source.

Had to move the custom error out of experimental to avoid a cycle. The real change is here.

NOTE: this only allows returning a custom error in the response, rather than an error_source. we still need a way to handle returning a custom error, outside of the response, that includes the error source

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 90 days. It will be closed in 2 weeks if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!