org.opendaylight.netconf.util.messages.NetconfMessageUtil#isErrorMessage(org.opendaylight.netconf.api.xml.XmlElement)

Method returns false if rpc-reply contains multiple error elements.

Netconf rfc permits multiple error elements.

https://tools.ietf.org/html/rfc6241#section-4.3

Because of this bug, netconf rpc service doesn't return parsed rpc-error list from reply, but fails with IllegalArgumentException when it tries to parse reply. Api user must parse exception message to get error list.