Clarify error message when file cannot be created,

The default error is "Not a valid SNDFILE* pointer." which is rather
obscure to end-users.

libs/audiographer/audiographer/sndfile/sndfile_writer.h

@@ -84,6 +84,10 @@ class SndfileWriter
 
 	virtual void init()
 	{
+		if (SF_ERR_NO_ERROR != SndfileHandle::error ()) {
+			throw Exception (*this, boost::str (boost::format
+						("Could create output file (%1%)") % path));
+		}
 		samples_written = 0;
 		add_supported_flag (ProcessContext<T>::EndOfInput);
 	}