fix: throw a nice error if config does not exist

src/config.ts

@@ -2,6 +2,7 @@ import { Client, checkKey } from './client.js';
 
 import fsp from 'node:fs/promises';
 import Ajv from 'ajv';
+import { existsSync } from 'node:fs';
 const ajv = new Ajv();
 
 const config_validate = ajv.compile({
@@ -30,6 +31,11 @@ export class Config {
 
     static async load(): Promise<Config> {
         const path = process.env.CONFIG_PATH ?? 'config.json';
+
+        if (!existsSync(path)) {
+            throw new Error(`The config file ${path} does not exist`);
+        }
+
         const raw = JSON.parse(
             await fsp.readFile(path, { encoding: 'utf8' }),
         ) as Config;