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/lib/async.js

@@ -0,0 +1,34 @@
+var defer = require('./defer.js');
+
+// API
+module.exports = async;
+
+/**
+ * Runs provided callback asynchronously
+ * even if callback itself is not
+ *
+ * @param   {function} callback - callback to invoke
+ * @returns {function} - augmented callback
+ */
+function async(callback)
+{
+  var isAsync = false;
+
+  // check if async happened
+  defer(function() { isAsync = true; });
+
+  return function async_callback(err, result)
+  {
+    if (isAsync)
+    {
+      callback(err, result);
+    }
+    else
+    {
+      defer(function nextTick_callback()
+      {
+        callback(err, result);
+      });
+    }
+  };
+}