Set an appropriate error if handshake fails

In the event of a failed handshake we set an error to notify the user.
To be able to signal consecutively failing connections, we need to reset
the error state after a disconnect, so if the new connection fails with
the same error, the error signal is emitted again.

Pick-to: 6.11
Change-Id: I0e6cca13c4dc7147ad9949bf0059273750c0d3de
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>