Return error strings instead of hard exiting

[rpreen]

At the moment there are various places where an error message is printed to stdout and exit(EXIT_FAILURE) is called. It would be better if a string containing an error message was returned by the function (or NULL if successful) so that the calling function can decide how to handle the error. By allowing this to propagate back up to the Python wrapper it could throw an exception with the error instead of terminating; stdout doesn't seem to be captured by jupyter notebooks for example.