elasticsearch-plugin

Describe the bug

Elastiknn exceptions do not extend ElasticsearchException, so they seem to always be turned into a 500, when a 4xx would often be more appropriate.

Expected behavior

• When a query is incompatible with a mapping, use an exception that will get converted to a 400 response code. [See code where exception is initialized.](https://github.com/alexklibisz/elastiknn/blob

How to reproduce?

Run go test ./... at the root in feat/es7 branch

You can easily see that a lot of test cases get failed, we need to fix it and make sure that all the test cases work as expected.

Additional Context

We recently upgraded the elastic search version from 6.x to 7.x which has some breaking changes, probably that might be the reason for some failed test cases.