node.js - grunt server doesn't start

I'm using yeoman, grunt & bower all together for the first time.

Following this tutorial and I stuck up where I couldn't the server didn't start and was throwing me a error

WANRNING: ENOENT, no such file or directory 'c:\wamp\www\yo\app\bower.json'

And I found the solution for that error over here and now I'm not getting error.

But after fixing that error and I ran the grunt serve command. And started getting this error (fewer or rare)

Running "concurrent:server" (concurrent) task
     Warning: Use --force to continue.

     Aborted due to warnings.

or this error (most occuring error)

Warning: grunt-cli: The grunt command line interface. (v0.1.13) Use --force to continue.

    Aborted due to warnings.

If I run grunt serve --force of course it's running with a yellow text as

Warning:   Used --force, continuing.

Someone help me out to over come this.

Thanks in advance.