when jekyll is not available, grunt doesn't print proper error

Created by: sazary

Hi
I tried to build bootstrap 3.1.1 using grunt, but jekyll was not installed on my machine. so running grunt in bootstrap directory told me:

Running "jekyll:docs" (jekyll) task
Warning: Please install Jekyll before running this task. Use --force to continue.

so I used grunt --force, and expected it to just skip compiling docs, but it printed this error and stopped:

Running "jekyll:docs" (jekyll) task
Warning: Please install Jekyll before running this task. Used --force, continuing.
Warning: Task "jekyll:docs" failed. Used --force, continuing.

Running "validation:files" (validation) task
_gh_pages/**/*.html:- No file is specified in the path!

and from the error, it wasn't obvious for me that the problem is jekyll. but installing jekyll solved the problem.
I'm completely noob in using these tools but I think it should not work this way.
just to mention that I downloaded source codes from https://github.com/twbs/bootstrap/archive/v3.1.1.zip , and I'm running these on ubuntu 12.04.