[server] fix unicode conversion capability in UndoException
By overriding the __unicode__ method, we ensure unicode builtin will return
Unicode object even for python versions that don't have a __unicode__ method by
default (python < 2.6)
In previous versions, when this method is not defined, string conversion is
attempted, and the result of string conversion is converted to Unicode using
the system default encoding which is ascii the most of the time.
###############################################################################
#
# Put here information about external resources / styles used by your cube,
# or to overides existing UI properties.
#
# Existing properties are available through the `sheet` dictionary available
# in the global namespace. You also have access to a `data` function which
# will return proper url for resources in the 'data' directory.
#
# /!\ this file should not be imported /!\
###############################################################################
# CSS stylesheets to include in HTML headers
# uncomment the line below to use template specific stylesheet
# STYLESHEETS = sheet['STYLESHEETS'] + [data('cubes.%(cubename)s.css')]