dtls: Handle errors/close_notify at all steps and propagate through the layers properly

dtls: Handle errors/close_notify at all steps and propagate through the layers properly

Previously we simply logged errors but never reported them to elements
or even to the user. Fatal errors are now properly reported.

Additionally proper connection closing is implemented based on EOS:
- dtlsenc: EOS will cause close_notify to be sent to the peer and only
           if the peer also sent back close_notify we will forward the
           EOS event.
- dtlsdec: EOS will be forwarded normally, this only means that the
           unterlying transport was closed. On receiving a DTLS packet
           containing close_notify, return EOS and send EOS downstream.