output "No synth app detected"  in non-synth-app directory

Thrown error in requiring `back/back-app` was not catched.
So error message was such a mess.

```
$ synth s

module.js:340
    throw err;
    ^
Error: Cannot find module '/home/kumagi/back/back-app'
  at Function.Module._resolveFilename (module.js:338:15)
  at Function.require.resolve (module.js:384:19)
  at requireUncached (/path/to/synth/synth/lib/requireUncached.js:16:14)
  at startServer (/path/to/synth/synth/lib/commands.js:65:15)
  at Object.exports.startServer (/path/to/synth/synth/lib/commands.js:95:5)
  at Object.<anonymous> (/path/to/synth/synth/bin/synth:64:12)
  at Module._compile (module.js:456:26)
  at Object.Module._extensions..js (module.js:474:10)
  at Module.load (module.js:356:32)
  at Function.Module._load (module.js:312:12)
  at Function.Module.runMain (module.js:497:10)
  at startup (node.js:119:16)
  at node.js:902:3
```

It should be

```
$ synth s
No synth app detected.
```

Thanks.