Error reporting from program running in VM

[tuommaki]

Current gRPC service & shim interface lacks support for proper error reporting from program running in VM.

While the error won't be visible in the JSON-RPC API nor in the blockchain, ultimately, it should be still logged by Gevulot node for debugging purposes.

Add reasonable error type with error code (int) and message (string) to gRPC service and corresponding shim APIs.

NOTE: Update GitBook's Program Development page with corresponding changes.