Change default Exception code to 500

Change default Exception code to 500

As a part of the fix for LP#1505153, the invalid
response of 0 was changed to 400 to fix a running
issue with the new version of WebOb.

It was pointed out after the fact  however that a
500 might be more appropriate here.

Additionally, other projects have implemented a 500
as the default so for the sake of consistency we should
consider doing the same.

This patch just changes the 400 to a 500 as the default
code.

Change-Id: Ie486dc49c927f9b50f07c1fc562e89c090924a40
Closes-Bug: #1505488