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/lodash/_defineProperty.js

@@ -0,0 +1,11 @@
+var getNative = require('./_getNative');
+
+var defineProperty = (function() {
+  try {
+    var func = getNative(Object, 'defineProperty');
+    func({}, '', {});
+    return func;
+  } catch (e) {}
+}());
+
+module.exports = defineProperty;