Bluetooth: Host: Add error to security changed callback

Add security error to security_changed callback. Call this callback when
security has failed and provide current security level and error.
Reason for failure can be.
 - Pairing procedure failed, pairing aborted before link encryption.
 - Link encrypt procedure failed
 - Link key refresh procedure failed.

Fix missing bt_conn_unref on encryption key refresh with error status.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>