return non-zero value when unsuccessfully exiting

`bash` treats a zero return value as a success. `gitc` still returns zero if it encounters an error during an operation.

    11:01:57 (rc-12350-backbonize-mediajs)
    ~/workspace/br/breport $ gitc pull
    
    Retrieving branch information from gitcycle.
    
    Fetching remote 'br'.
    
    
    Merging remote branch 'rc' from 'br/breport'.
    
    error: Your local changes to the following files would be overwritten by merge:
    	public/javascripts/backbone/views/media_video_view.js
    Please, commit your changes or stash them before you can merge.
    Aborting
    
    
    Gitcycle encountered an error when running the last command:
      git merge br/rc
    Please copy this session's output and send it to gitcycle@bleacherreport.com.
    
    
    11:02:04 (rc-12350-backbonize-mediajs)
    ~/workspace/br/breport $ echo $?
    0

 This means that chaining commands like `gitc pull && bundle` doesn't end up doing what you'd expect.