Spark web UI return `HTTP ERROR 500` when GET arguments is missing.

Author: Kirill A. Korinskiy <catap@catap.ru>

Closes #4239 from catap/ui_500 and squashes the following commits:

520e180 [Kirill A. Korinskiy] [SPARK-5672][Web UI] Return `HTTP ERROR 400` when have missing args