Changed AddDataChangeCallback to mirror python-opcua implementation

[Magnulas]

The AddDataChangeCallback function was changed to return a OPC UA status code to indicate if adding the callback was successful. The exception that was thrown from AddDataChangeCallback when adding a callback failed triggered a goodbye message to be sent to the connecting client when they were attempting to create a monitored item. This fix should make connecting clients receive the proper OPC UA status code instead.

[ralf1070]

At most places freeOpcUa indicates errors by throwing exceptions. This is done for a reason as you are able to separate error handling from normal program flow. You don't need to handle an error at the same place as you handle a regular result of a function. And - for what I think - this is the usual way to indicate errors in C++. If you need further information about the thrown error then subclass "std::runtime_error" and add the information you need and throw such an instance.

From my point of view returning status codes is good old K&R C style and should be avoided where ever possible. To create temporary instances of pairs of return values and status codes is even worse. It is inefficient and confusing ... just my 2 cents.

[Magnulas]

If preferred I'll submit a pull request where AddDataChangeCallback throws a exception wrapping the appropriate status code and let CreateMonitoredItem handle the exception, setting the status code for the request based on what the exception contained.

[ralf1070]

From my point of view this would be the better solution.