Fix NRT file paths

Addresses:

- Score:recordNRT forced the user to generate her own osc file path, which is a pointless inconvenience. (A further refinement here would be to delete the temp file automatically.)

- Object:render used a relative path for the osc score file, relying on sclang's working directory to be sensible. That was never safe.

- Score threw confusing errors if it was unable to write a file. Now it will throw proper, descriptive errors.