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/_overArg.js

@@ -0,0 +1,15 @@
+/**
+ * Creates a unary function that invokes `func` with its argument transformed.
+ *
+ * @private
+ * @param {Function} func The function to wrap.
+ * @param {Function} transform The argument transform.
+ * @returns {Function} Returns the new function.
+ */
+function overArg(func, transform) {
+  return function(arg) {
+    return func(transform(arg));
+  };
+}
+
+module.exports = overArg;