Look for 'caused_by.reason' in ES error response
When the ES response does not contain `reason` or `root_cause[0].reason`
is empty, there is no information for the user to know what is going
wrong.
An example of the error message after this change:
```
Failed to evaluate queries and expressions: failed to execute query A: Trying to create too many buckets. Must be less than or equal to: [65536] but this number of buckets was exceeded. This limit can be set by changing the [search.max_buckets] cluster level setting.
```
Related to https://github.com/grafana/grafana/issues/61246
Ruud van der Weijde
b44df6c8d1