index: throw error if `start` is not defined.

lib/index.js

@@ -37,6 +37,11 @@ export default class Now extends EventEmitter {
       throw new Error(`Failed to read JSON in "${path}/package.json"`);
     }
 
+    if (!pkg.scripts || !pkg.scripts.start) {
+      throw new Error('Missing `start` script in `package.json`. ' +
+        'See: https://docs.npmjs.com/cli/start.');
+    }
+
     if (this._debug) console.time('> [debug] Getting files');
     const files = await getFiles(path, pkg, { limit: ONEMB, debug: this._debug });
     if (this._debug) console.timeEnd('> [debug] Getting files');