Map Elastiknn Exception types to the appropriate HTTP status codes

[alexklibisz]

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.
• When a vector has incorrect dimensions, use an exception that will get converted to a 400 response code. See code where exception is initialized.
• This should all be tested, e.g., in the VectorMapperSuite.

(There are probably some other places where this is the case)

Environment (please complete the following information):

• Elastiknn version: All
• OS: All

To Reproduce

• Attempt to run a query that is incompatible with a given mapping.
• Attempt to index a vector of the wrong dimensionality.

Additional context

• I stumbled onto this issue while fixing Null Pointer Exception when searching by vector Id for an Id that doesn't exist #310 .
• Elasticsearch has an ElasticsearchException, which seems to be the intended way to specify the severity of an exception and thus its eventual status code.