exit on no app configured

execute `avoscloud` on non app folder will raise this error:
```
➜  ~  avoscloud
[INFO]: Cloud Code Project Home Directory: /Users/asaka/
[INFO]: You are not in a app.Please checkout <app>

/usr/local/lib/node_modules/avoscloud-code/bin/avoscloud:559
        throw "Can't find a valid app to execute command '" + CMD + "'.";
                                                                  ^
Can't find a valid app to execute command 'undefined'.
```

should do `exit(1)` on this branch.