Always use unicode in KimchiException message

When KimchiException was raised inside a cherrypy application it returned
a unicode message. But if the exception was raised independently the cherrypy
server (ie, using model instance directly) it returned a string message.
To keep the same behavior in all those situations, let KimchiException
always returns a unicode message

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>