Better api error handling (#790)

* API generation

* Updated code generation

* Updated test

api/index.js

@@ -26,6 +26,9 @@ function ESAPI (opts) {
   assert(opts.ConfigurationError, 'Missing ConfigurationError class')
   assert(opts.result, 'Missing default result object')
 
+  const { result } = opts
+  opts.handleError = handleError
+
   const apis = {
     bulk: lazyLoad('bulk', opts),
     cat: {
@@ -480,6 +483,11 @@ function ESAPI (opts) {
   }
 
   return apis
+
+  function handleError (err, callback) {
+    if (callback) return callback(err, result)
+    return Promise.reject(err)
+  }
 }
 
 // It's unlikely that a user needs all of our APIs,