feat(mocks): make $timeout#flush throw an exception when empty

When calling $timeout.flush with or without a delay an exception should
be thrown if there is nothing to be flushed.

This prevents tests from flushing stuff unnecessarily.

BREAKING CHANGE: calling $timeout.flush(delay) when there is no task to be flushed
within the delay throws an exception now.

Please adjust the delay or remove the flush call from your tests as the exception
is a signed of a programming error.