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/trim-right/index.js

@@ -0,0 +1,10 @@
+'use strict';
+module.exports = function (str) {
+	var tail = str.length;
+
+	while (/[\s\uFEFF\u00A0]/.test(str[tail - 1])) {
+		tail--;
+	}
+
+	return str.slice(0, tail);
+};