Add ElasticsearchException to model Elasticsearch exceptions

[russcam]

This commit adds an ElasticsearchException struct that models exceptions returned by Elasticsearch in the event of an error.

An exception() function is exposed on Response to deserialize the response body into ElasticsearchException when the status code is in the 400-599 range.

Consideration was made to exposing this on the Error struct, performing the deserialization inside error_for_status_code, but this introduces some awkwardness into the API:

• error_for_status_code would ideally be async because the .json() call is async
• .json() consumes the response, which would not be possible for error_for_status_code_ref which has a shared reference to Response.

Additional details about an exception are collected in a map and exposed through additional_details function.

Closes #112

[russcam]

Think we can combine Error and Cause into the same type

[russcam]

Decided to keep Error and Cause separate for now. They both map to ElasticsearchException on the server side, but it looks like certain properties are only ever exposed on the top level exception which is deserialized to Error e.g. root_cause, header and failed_shards (which is not currently mapped in Error).

[mwilliammyers]

I haven't read this in great detail, but should it be Source instead of Cause, to mirror the std::error::Error trait?

Or is it just root_cause()?

[russcam]

I haven't read this in great detail, but should it be Source instead of Cause, to mirror the std::error::Error trait?

I think you might be referring to the previous comment about Error and Cause https://github.com/elastic/elasticsearch-rs/pull/118#issuecomment-666071878? The Error in this case is a struct that models the error field returned on an exception, for example,

{
  "error": {
    "root_cause": [{
      "type": "index_not_found_exception",
      "reason": "no such index [test_index]",
      "resource.type": "index_or_alias",
      "resource.id": "test_index",
      "index_uuid": "_na_",
      "index": "test_index"
    }],
    "type": "index_not_found_exception",
    "reason": "no such index [test_index]",
    "resource.type": "index_or_alias",
    "resource.id": "test_index",
    "index_uuid": "_na_",
    "index": "test_index"
  },
  "status": 404
}

is a response when an index does not exist. The error field is a serialized exception from Elasticsearch and in this case, contains a root_cause field, but other types of exception responses can contain a caused_by field that is also a serialized exception from Elasticsearch. Rather than reusing the Error struct as the type for the caused_by field though, there's a Cause struct to model it because it looks like certain properties are only ever exposed on the top level exception which is deserialized to Error and never on caused_by.