only track preprocessor errors if raven_client exists

Author: takempf

lib/preprocessor.js

@@ -34,11 +34,13 @@ var Preprocessor = function( preprocessor_path, options ){
 			// so log them here
 			if( err ){
 				logger.log( 'Preprocessor Error:\n'+ err.stack, 0 );
-				raven_client.captureError( err, {
-					extra: {
-						context: context
-					}
-				});
+				if( raven_client ){
+					raven_client.captureError( err, {
+						extra: {
+							context: context
+						}
+					});
+				}
 				if( callback ) return callback( context );
 			}
 			if( callback ) return callback( preprocessed_context );