Endpoint decorator unicode exception

This seems to fix it.  @manthey Maybe you can suggest a test case where `repr` is needed.  It is a bit complicated by the fact that the value is an object.
Since this is in our fall-back error handler, I don't think it is necessary to make the results look as good as possible.  Therefore, the original could just be changed to repr(value), like so:
`val = {'message': '%s: %s' % (t.__name__, repr(value)),`