Fixes the error in `test_onErrorCallback`.

The test testCMFActivity.TestCMFActivity.test_onErrorCallback was failing because some weak references were not dropped. The reason was that _DequeueMessageException was declared as global and it kept a traceback including the variables pointed by the weak references, preventing their garbage collection. This was fixed by using a class for the _DequeueMessageException instead of a singleton, and creating a new instance on every raise.

The test pass now in local and a test run is in process.