Updated abort behavior (#1141)

* Updated abort behavior

- Support for aborting a request with the promise api
- Aborting a request will cause a RequestAbortedError
- Normalized Connection class errors, now every error returned is
wrapped by the client errors constructors

* Updated test

* Updated docs

* Updated code generation script

* Renamed test

* Code coverage

* Avoid calling twice transport.request

test/behavior/sniff.test.js

@@ -210,7 +210,7 @@ test('Sniff on connection fault', t => {
     class MyConnection extends Connection {
       request (params, callback) {
         if (this.id === 'http://localhost:9200/') {
-          callback(new Error('kaboom'), null)
+          callback(new errors.ConnectionError('kaboom'), null)
           return {}
         } else {
           return super.request(params, callback)