[fix] handle monitor error, and make `forever stop(all)` peaceful

This PR fixed below behaviours:

`app1.js`:
```javascript
console.log(Date.now());
```

`app2.js`
```javascript
setInterval(function(){
  console.log(Date.now());
}, 1000);
```

run commands:
```
$ forever start app1.js
$ forever start app2.js
$ forever list
```
We'll notice that `app1.js` has been marked as `STOPPED`, 'Cause the script has been executed and exited.  Okay, let's try:
```
$ forever stopall
```
Before this PR, the CLI will exit without information, Ops, when we look into the logs, we can found that `an unhandled error` was caught, it makes CLI crashing.

After this PR, every thing will goes correct!!!