More details in error response from server #11

It would be very useful for the client if, in case of an error on the server side, the server, in addition to the result code FF, also sent detail why command/commands failed. Currently, in case of failure, response ends with the result code FF and provides zero information for tracking issue. I suggest that the following information should be added at the end after result code:

• index of the command in the request that failed (the index of course starts with 0), the type would be [ uint32 ]
• error message containing a description of the error, the type would be [ uint32 size, char* text ] like for other text responses

Error message can be set as optional in case that the emulator can not provide meaningful error message, but it may be better then emulator returns empty error message (size = 0) in such case.