If an exception is thrown before the callback is registered,
the finally{} block will fail with an exception explaining
the callback can't be unregistered. This masks the actual
exception.
This will ensure the original exception is printed instead.
Test: ConnectivityDiagnosticsManagerTest
Change-Id: I38aba86ff6d5d2a2a8a27c389feffd3aab788774
5b2f852fcf