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/defer.js

@@ -0,0 +1,26 @@
+module.exports = defer;
+
+/**
+ * Runs provided function on next iteration of the event loop
+ *
+ * @param {function} fn - function to run
+ */
+function defer(fn)
+{
+  var nextTick = typeof setImmediate == 'function'
+    ? setImmediate
+    : (
+      typeof process == 'object' && typeof process.nextTick == 'function'
+      ? process.nextTick
+      : null
+    );
+
+  if (nextTick)
+  {
+    nextTick(fn);
+  }
+  else
+  {
+    setTimeout(fn, 0);
+  }
+}