BZ#990382: Return error message and code when wrong gear id is given to oo-pam-enable/disable

Testing:

Before:
```
[root@xxx ~]# oo-pam-enable -c 123456789
[root@xxx ~]# echo $?
0
```

After this patch:

```
[root@xxx ~]# oo-pam-enable -c 123456789
can't find user for 123456789
[root@xxx ~]# echo $?
255

```