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/multiply.js

@@ -0,0 +1,22 @@
+var createMathOperation = require('./_createMathOperation');
+
+/**
+ * Multiply two numbers.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.7.0
+ * @category Math
+ * @param {number} multiplier The first number in a multiplication.
+ * @param {number} multiplicand The second number in a multiplication.
+ * @returns {number} Returns the product.
+ * @example
+ *
+ * _.multiply(6, 4);
+ * // => 24
+ */
+var multiply = createMathOperation(function(multiplier, multiplicand) {
+  return multiplier * multiplicand;
+}, 1);
+
+module.exports = multiply;