Dashboard widgets with error information should include error code/identifier

[douglascamata]

Many of the dashboard widgets with error information in this configuration are using a hardcoded "errors" caption in the charts, like so (from the Observatorium API dashboard):

This hides the details about which errors is this. It would be greatly useful if we could see the error code or any other error attribute that helps identifying it:

This happens in Observatorium's and Thanos' dashboards, across gRPC and HTTP errors charts.