check if chroot exist for specified project
    
    addressing:
    Exception on /api/coprs/hhorak/mysql55-test1/detail/fedora-20-x86_64/ [GET]
    Traceback (most recent call last):
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
        response = self.full_dispatch_request()
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
        rv = self.handle_user_exception(e)
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
        rv = self.dispatch_request()
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
        return self.view_functions[rule.endpoint](**req.view_args)
      File "/usr/share/copr/coprs_frontend/coprs/views/api_ns/api_general.py", line 414, in copr_chroot_details
        output = {'output': 'ok', 'buildroot_pkgs': ch.buildroot_pkgs}
    AttributeError: 'NoneType' object has no attribute 'buildroot_pkgs'