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/es-abstract/helpers/assign.js

@@ -0,0 +1,17 @@
+var bind = require('function-bind');
+var has = bind.call(Function.call, Object.prototype.hasOwnProperty);
+
+var $assign = Object.assign;
+
+module.exports = function assign(target, source) {
+	if ($assign) {
+		return $assign(target, source);
+	}
+
+	for (var key in source) {
+		if (has(source, key)) {
+			target[key] = source[key];
+		}
+	}
+	return target;
+};