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/_baseIsNaN.js

@@ -0,0 +1,12 @@
+/**
+ * The base implementation of `_.isNaN` without support for number objects.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is `NaN`, else `false`.
+ */
+function baseIsNaN(value) {
+  return value !== value;
+}
+
+module.exports = baseIsNaN;