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/_stringToArray.js

@@ -0,0 +1,18 @@
+var asciiToArray = require('./_asciiToArray'),
+    hasUnicode = require('./_hasUnicode'),
+    unicodeToArray = require('./_unicodeToArray');
+
+/**
+ * Converts `string` to an array.
+ *
+ * @private
+ * @param {string} string The string to convert.
+ * @returns {Array} Returns the converted array.
+ */
+function stringToArray(string) {
+  return hasUnicode(string)
+    ? unicodeToArray(string)
+    : asciiToArray(string);
+}
+
+module.exports = stringToArray;