Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/asynckit/stream.js

@@ -0,0 +1,21 @@
+var inherits              = require('util').inherits
+  , Readable              = require('stream').Readable
+  , ReadableAsyncKit      = require('./lib/readable_asynckit.js')
+  , ReadableParallel      = require('./lib/readable_parallel.js')
+  , ReadableSerial        = require('./lib/readable_serial.js')
+  , ReadableSerialOrdered = require('./lib/readable_serial_ordered.js')
+  ;
+
+// API
+module.exports =
+{
+  parallel      : ReadableParallel,
+  serial        : ReadableSerial,
+  serialOrdered : ReadableSerialOrdered, 
+};
+
+inherits(ReadableAsyncKit, Readable);
+
+inherits(ReadableParallel, ReadableAsyncKit);
+inherits(ReadableSerial, ReadableAsyncKit);
+inherits(ReadableSerialOrdered, ReadableAsyncKit);