Add option to make exception format errors fatal.
Add option to make exception format errors fatal.

Adds a new fatal_exception_format_errors config option which
defaults to False. This option is use to control how the base
NovaException class handles errors which can occur when it
formats error messages.

The motivation for this change is to be able to enable exception
format checking in our tests by setting
fatal_exception_format_errors=True.

Change-Id: Ie96261ed96bcede4a2b5ec5600cb93c15141a800

n0ano/gantt,n0ano/gantt