Add warning when config parsing fails

Summary:
This can quite easily happen if you manually edit it and
aren't careful. The least we can do is tell the user about it,
because it be a bit surprising otherwise.

Reviewed By: jknoxville

Differential Revision: D17626760

fbshipit-source-id: 8c7d64b041fa4b97ce2753116800d0e855442dd9

static/setup.js

@@ -41,6 +41,8 @@ module.exports = function(argv) {
     };
   } catch (e) {
     // file not readable or not parsable, overwrite it with the new config
+    console.warn(`Failed to read ${configPath}: ${e}`);
+    console.info('Writing new default config.');
     fs.writeFileSync(configPath, JSON.stringify(config));
   }