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/capitalize.js

@@ -0,0 +1,23 @@
+var toString = require('./toString'),
+    upperFirst = require('./upperFirst');
+
+/**
+ * Converts the first character of `string` to upper case and the remaining
+ * to lower case.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.0.0
+ * @category String
+ * @param {string} [string=''] The string to capitalize.
+ * @returns {string} Returns the capitalized string.
+ * @example
+ *
+ * _.capitalize('FRED');
+ * // => 'Fred'
+ */
+function capitalize(string) {
+  return upperFirst(toString(string).toLowerCase());
+}
+
+module.exports = capitalize;