Improve response error message (#1457)

lib/errors.js

@@ -90,7 +90,15 @@ class ResponseError extends ElasticsearchClientError {
     super('Response Error')
     Error.captureStackTrace(this, ResponseError)
     this.name = 'ResponseError'
-    this.message = (meta.body && meta.body.error && meta.body.error.type) || 'Response Error'
+    if (meta.body && meta.body.error && meta.body.status) {
+      if (Array.isArray(meta.body.error.root_cause)) {
+        this.message = meta.body.error.root_cause.map(entry => `[${entry.type}] Reason: ${entry.reason}`).join('; ')
+      } else {
+        this.message = 'Response Error'
+      }
+    } else {
+      this.message = 'Response Error'
+    }
     this.meta = meta
   }
 
@@ -108,6 +116,10 @@ class ResponseError extends ElasticsearchClientError {
   get headers () {
     return this.meta.headers
   }
+
+  toString () {
+    return JSON.stringify(this.meta.body)
+  }
 }
 
 class RequestAbortedError extends ElasticsearchClientError {