Message instances created by oslo_i18n are subclasses of the Unicode
type (unicode on Python 2, str on Python 3) and have no __unicode__()
method. exception_to_unicode() raises an AttributeError when trying to
convert it to Unicode.
This change fixes this issue and adds an unit test.
Change-Id: Ica67429ac64f74e5c636b6d74d71910a26511378