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/fp/convert.js

@@ -0,0 +1,18 @@
+var baseConvert = require('./_baseConvert'),
+    util = require('./_util');
+
+/**
+ * Converts `func` of `name` to an immutable auto-curried iteratee-first data-last
+ * version with conversion `options` applied. If `name` is an object its methods
+ * will be converted.
+ *
+ * @param {string} name The name of the function to wrap.
+ * @param {Function} [func] The function to wrap.
+ * @param {Object} [options] The options object. See `baseConvert` for more details.
+ * @returns {Function|Object} Returns the converted function or object.
+ */
+function convert(name, func, options) {
+  return baseConvert(util, name, func, options);
+}
+
+module.exports = convert;