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/uniqueId.js

@@ -0,0 +1,28 @@
+var toString = require('./toString');
+
+/** Used to generate unique IDs. */
+var idCounter = 0;
+
+/**
+ * Generates a unique ID. If `prefix` is given, the ID is appended to it.
+ *
+ * @static
+ * @since 0.1.0
+ * @memberOf _
+ * @category Util
+ * @param {string} [prefix=''] The value to prefix the ID with.
+ * @returns {string} Returns the unique ID.
+ * @example
+ *
+ * _.uniqueId('contact_');
+ * // => 'contact_104'
+ *
+ * _.uniqueId();
+ * // => '105'
+ */
+function uniqueId(prefix) {
+  var id = ++idCounter;
+  return toString(prefix) + id;
+}
+
+module.exports = uniqueId;