[SPARK-5729] Potential NPE in standalone REST API

If the user specifies a bad REST URL, the server will throw an NPE instead of propagating the error back. This is because the default `ErrorServlet` has the wrong prefix. This is a one line fix. I am will add more comprehensive tests in a separate patch.

Author: Andrew Or <andrew@databricks.com>

Closes #4518 from andrewor14/rest-npe and squashes the following commits:

16b15bc [Andrew Or] Correct ErrorServlet context prefix