demeteorizer should exit with errror code if error occurs

i am using demeteorizer in gruntjs.
when demeteorizer fails an exit  code different from 0 should be returned in order to inform the caller about the error.
with this changes, everything works as expected

here is the gruntjs code:

```
shell: {
	    	demeteorizer : {
	    		command : 'demeteorizer',
	    		options: {
	    			execOptions: {
                    	cwd: '<%= path  %>',
                    	failOnError: true

                	}
	    		}
	    	}
	    },
```
and how gruntjs stops when demeteorizer returns  error code 1
```
myo@pingu:~/Dev/code/XX/tools/deploy$ grunt shell
Running "shell:demeteorizer" (shell) task
Input: /home/myo/Dev/code/XX/src/XXX
Output: /home/myo/Dev/code/XX/src/XXX/.demeteorized
Determining Meteor version...
Meteor version: 0.9.1.1

Output folder exists, deleting...
ERROR: Error: EACCES, unlink '/home/myo/Dev/code/XX/src/XXX/.demeteorized/programs/server/node_modules/.bin/semver'
Warning: Command failed:  Use --force to continue.

Aborted due to warnings.

```