Add _FATAL_EXCEPTION_FORMAT_ERRORS global.

Add a new global variable to control when exception format errors
are fatal. The Glance functional/unit test now make use of this
to ensure exceptions are correctly formatted when testing.

Change-Id: I720336e10345a0e33937c7241c37f548d6f87ea2
Merge "Add _FATAL_EXCEPTION_FORMAT_ERRORS global."