Implement EntityDiscoveryServer.ReportError according to new spec recommendations

[marcellanz]

As discussed on todays constributors call, ReportError should:

• Should be used for informational purposes only—the message MUST be reported to the developer. Not logged as DEBUG. Typically that would mean writing it to STDOUT but it may go somewhere else depending on the language used.
• If stream is closed afterwards, then it should passivate the entity (just as normal)

therefore, if the proxy does not close the stream, the entity will receive further commands/events for the entity. The user (developer) should receive the error and then decides what to do. It is advised to make the error discoverable, most probably through writing it to stderr/stdout.