I think this is a regression on master, but it's also possible that this never behaved the way I remember. I was testing out some of the new Metricbeat 7.11 dashboards that include Lens visualizations, and found this one:

As you can see, we are getting an error message from esaggs instead of an error from the dimension level. It would be better to have a human-readable error instead.

Steps to reproduce:

1. Create an index mapping that contains a field, but don't add any documents
2. Use the empty field in a Lens visualization
3. Delete the field from the index mapping
4. Load the Lens visualization