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/is-generator-fn/index.js

@@ -0,0 +1,14 @@
+'use strict';
+const {toString} = Object.prototype;
+
+module.exports = value => {
+	if (typeof value !== 'function') {
+		return false;
+	}
+
+	return (value.constructor && value.constructor.name === 'GeneratorFunction') ||
+		toString.call(value) === '[object GeneratorFunction]';
+};
+
+// TODO: Remove this for the next major release
+module.exports.default = module.exports;