Add verification required error code

Author: Steven-Chan

skygear/src/main/java/io/skygear/skygear/Error.java

@@ -204,6 +204,10 @@ public static enum Code {
          * User disabled error.
          */
         USER_DISABLED(127),
+        /**
+         * Verification required error.
+         */
+        VERIFICATION_REQUIRED(128),
         /**
          * Unexpected error.
          */