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/lte.js

@@ -0,0 +1,30 @@
+var createRelationalOperation = require('./_createRelationalOperation');
+
+/**
+ * Checks if `value` is less than or equal to `other`.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.9.0
+ * @category Lang
+ * @param {*} value The value to compare.
+ * @param {*} other The other value to compare.
+ * @returns {boolean} Returns `true` if `value` is less than or equal to
+ *  `other`, else `false`.
+ * @see _.gte
+ * @example
+ *
+ * _.lte(1, 3);
+ * // => true
+ *
+ * _.lte(3, 3);
+ * // => true
+ *
+ * _.lte(3, 1);
+ * // => false
+ */
+var lte = createRelationalOperation(function(value, other) {
+  return value <= other;
+});
+
+module.exports = lte;