sPFShared/src/main/java/it/polimi/spf/shared/model/InvocationResponse.java
public static InvocationResponse error(String errorMessage) {
		InvocationResponse resp = new InvocationResponse(ERROR);
		resp.errorMessage = errorMessage;
		return resp;
	}
public static InvocationResponse error(String errorMessage) {
		InvocationResponse resp = new InvocationResponse(ERROR);
		resp.errorMessage = errorMessage;
		return resp;
	}
Creates a new {@link InvocationResponse} to hold the error that occurred
during the execution of the service.

@param errorMessage
- the error message that describes the occurred error.
@return the invocation request.