Remove test_describe_fail() and add tests for describe() on custom rendarables

_describe() checks with:

   try:
       len(words)
   except AttributeError:
       # WithProperties and Property don't have __len__
       return ["???"]

if passed command is some kind of rendarable, so describe() should never fail.

But this doesn't work for renderables which are instances of new-style
classes - add failing tests for that cases.
Fix _describe() when command is new-style class instance
Merge branch 'fix_custom_renderables_describe' of git://github.com/vrutsky/buildbot