Fix bug detected by unit-test.

Reset m_currentOperation when return error, otherwise processiTIPMessage
can't be called anymore.

Also made the return error be returned async, when processiTIPMessage()
is no longer on the stack. During a successfull operation that was the case,
but on error cases we were emitting immediately. Callers calling this method in
sync mode with nested event loop (due to kmail memento
limitation) couldn't catch the signal, as they only started listening
to the signal after calling processiTIPMessage().