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/add.js

@@ -0,0 +1,22 @@
+var createMathOperation = require('./_createMathOperation');
+
+/**
+ * Adds two numbers.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.4.0
+ * @category Math
+ * @param {number} augend The first number in an addition.
+ * @param {number} addend The second number in an addition.
+ * @returns {number} Returns the total.
+ * @example
+ *
+ * _.add(6, 4);
+ * // => 10
+ */
+var add = createMathOperation(function(augend, addend) {
+  return augend + addend;
+}, 0);
+
+module.exports = add;