Improve errors (#893)

- `RippledError`: Include the full response from the `rippled` server.
- A new test ensures correct behavior when `streams` is not an array.
- `NotConnectedError` may be thrown with a different message than before.

test/fixtures/rippled/index.js

@@ -16,6 +16,7 @@ module.exports = {
   },
   empty: require('./empty'),
   subscribe: require('./subscribe'),
+  subscribe_error: require('./subscribe_error'),
   unsubscribe: require('./unsubscribe'),
   account_objects: {
     normal: require('./account-objects'),

test/fixtures/rippled/subscribe_error.json

@@ -0,0 +1,13 @@
+{
+  "id": 0,
+  "status": "error",
+  "type": "response",
+  "error": "invalidParams",
+  "error_code": 31,
+  "error_message": "Invalid parameters.",
+  "request": {
+    "command": "subscribe",
+    "id": 0,
+    "streams": "validations"
+  }
+}