essql errors just show "check server logs"

[w33ble]

Kibana version: 6.7+

Describe the bug:

When you put an invalid query in the essql function, like querying an non-existent index or try to use a field that doesn't exist, all you see is this:

[essql] > See server logs for details.

But I'm a user, I can't see the logs, which are actually super helpful:

Error: Unexpected error from Elasticsearch: [verification_exception] Found 1 problem(s)
line 1:22: Unknown index [not_an_index]

We should surface the error message in the app, otherwise it's nearly impossible to figure out what you did wrong.

This is also a regression, in 6.6 we did show the error we got from Elasticsearch.

Steps to reproduce:

1. Create an element with this expression: essql query="select username from not_an_index"

[elasticmachine]

Pinging @elastic/kibana-canvas