Fix flakey test that causes issues with sharedb-mongo in Travis

[nateps]

Changes

• Fix flakey test that failed frequently in Travis when run against sharedb-mongo, but didn't fail locally. The tests causes the client to send multiple fetches at the same time in order to get missing ops, and one of the fetches would attempt to getOps after the test done() had been called and the database connection had been closed. Waiting for whenNothingPending() before calling done resolves the flakiness.
• Add 'error' handlers that pass the error to done(), so that the problematic test can be identified
• Remove callback existence checks in PubSubMemory that are not needed, because sharedb will never call the methods without a callback (more accurate coverage testing)

[coveralls]

Coverage remained the same at 96.735% when pulling f2b9a40 on test-error-visibility into 66e8866 on master.