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/isNaN.js

@@ -0,0 +1,38 @@
+var isNumber = require('./isNumber');
+
+/**
+ * Checks if `value` is `NaN`.
+ *
+ * **Note:** This method is based on
+ * [`Number.isNaN`](https://mdn.io/Number/isNaN) and is not the same as
+ * global [`isNaN`](https://mdn.io/isNaN) which returns `true` for
+ * `undefined` and other non-number values.
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Lang
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is `NaN`, else `false`.
+ * @example
+ *
+ * _.isNaN(NaN);
+ * // => true
+ *
+ * _.isNaN(new Number(NaN));
+ * // => true
+ *
+ * isNaN(undefined);
+ * // => true
+ *
+ * _.isNaN(undefined);
+ * // => false
+ */
+function isNaN(value) {
+  // An `NaN` primitive is the only value that is not equal to itself.
+  // Perform the `toStringTag` check first to avoid errors with some
+  // ActiveX objects in IE.
+  return isNumber(value) && value != +value;
+}
+
+module.exports = isNaN;