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/tap.js

@@ -0,0 +1,29 @@
+/**
+ * This method invokes `interceptor` and returns `value`. The interceptor
+ * is invoked with one argument; (value). The purpose of this method is to
+ * "tap into" a method chain sequence in order to modify intermediate results.
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Seq
+ * @param {*} value The value to provide to `interceptor`.
+ * @param {Function} interceptor The function to invoke.
+ * @returns {*} Returns `value`.
+ * @example
+ *
+ * _([1, 2, 3])
+ *  .tap(function(array) {
+ *    // Mutate input array.
+ *    array.pop();
+ *  })
+ *  .reverse()
+ *  .value();
+ * // => [2, 1]
+ */
+function tap(value, interceptor) {
+  interceptor(value);
+  return value;
+}
+
+module.exports = tap;