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/toUpper.js

@@ -0,0 +1,28 @@
+var toString = require('./toString');
+
+/**
+ * Converts `string`, as a whole, to upper case just like
+ * [String#toUpperCase](https://mdn.io/toUpperCase).
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category String
+ * @param {string} [string=''] The string to convert.
+ * @returns {string} Returns the upper cased string.
+ * @example
+ *
+ * _.toUpper('--foo-bar--');
+ * // => '--FOO-BAR--'
+ *
+ * _.toUpper('fooBar');
+ * // => 'FOOBAR'
+ *
+ * _.toUpper('__foo_bar__');
+ * // => '__FOO_BAR__'
+ */
+function toUpper(value) {
+  return toString(value).toUpperCase();
+}
+
+module.exports = toUpper;