Need ability to report errors in response to Command and Connection Settings Offer

[tigrannajaryan]

The Server can send a command or a connection settings offers to the Agent. There is currently no good way for the Agent to respond back with an error if something goes wrong while processing these messages from the Server.

It may be worth figuring out how in general the Agent should be able to report errors to the Server if the error is a result of a processing of a message from the Server. Should the Agent be able to tell which message exactly failed processing? Do we need message ids if so?

I think this needs to be figured out before marking the spec Stable since it may be needed for capabilities which we intend to mark Stable.

See also discussion here open-telemetry/opamp-go#186