Check if build is cancellable in client

Bugzilla - https://bugzilla.redhat.com/show_bug.cgi?id=1171673
Yes, why can't I cancel a new build?  Also, canceling an already cancelled build should not be an error.  


> On Dec 9, 2014, at 9:24 AM, Andy Goldstein <notifications@github.com> wrote:
> 
> In pkg/cmd/cli/cmd/cancelbuild.go:
> 
> > @@ -81,3 +77,14 @@ Examples:
> >  	cmd.Flags().Bool("restart", false, "Specify if the build should be restarted after it's cancelled.")
> >  	return cmd
> >  }
> > +
> > +// checkBuildIsCancellable checks if another cancellation event was triggered, and if the build status is correct.
> > +func checkBuildIsCancellable(build *buildapi.Build) {
> > +	if build.Status != buildapi.BuildStatusPending && build.Status != buildapi.BuildStatusRunning {
> > +		glog.Fatalf("A build can be cancelled only if it has new/pending/running status.")
> The error message doesn't line up 100% with the if conditions. You aren't checking for BuildStatusNew - do you need to be?
> 
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