object-storage: Check for cancellation before resulting in assert failure

As per doumentation: "It is a programming error to create an identical proxy while asynchronously creating one. Not cancelling this operation will result in an assertion failure when calling cc_object_storage_create_dbus_proxy_finish()." In order to fullfill the second part we need to check for errors (including cancellation ones) before we generate an assertion failure.

Fixes: #158 (closed)