Be nice(r) when no arguments given

Having a standalone install `persistgraphql` gives

> Usage: persistgraphql input_file [output_file]
(node:46389) UnhandledPromiseRejectionWarning: TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be one of type string, Buffer, or URL. Received type undefined
    at Object.fs.stat (fs.js:884:3)
    at /usr/local/lib/node_modules/persistgraphql/lib/src/ExtractGQL.js:45:16
    at new Promise (<anonymous>)
    at Function.ExtractGQL.isDirectory (/usr/local/lib/node_modules/persistgraphql/lib/src/ExtractGQL.js:44:16)
    at /usr/local/lib/node_modules/persistgraphql/lib/src/ExtractGQL.js:139:24
    at new Promise (<anonymous>)
    at ExtractGQL.readInputPath (/usr/local/lib/node_modules/persistgraphql/lib/src/ExtractGQL.js:138:16)
    at /usr/local/lib/node_modules/persistgraphql/lib/src/ExtractGQL.js:129:19
    at new Promise (<anonymous>)
    at ExtractGQL.processInputPath (/usr/local/lib/node_modules/persistgraphql/lib/src/ExtractGQL.js:128:16)

src/ExtractGQL.ts

@@ -342,6 +342,7 @@ export const main = (argv: YArgsv) => {
 
   if (args.length < 1) {
     console.log('Usage: persistgraphql input_file [output_file]');
+    process.exit(1);
   } else if (args.length === 1) {
     inputFilePath = args[0];
   } else {