Ensure that the appended categories does in fact exist.
Ensure that the appended categories does in fact exist.

Closes #43

alvarolopez/occi-os,alvarolopez/occi-os,EGI-FCTF/occi-os,tmetsch/occi-os,EGI-FCTF/occi-os,enolfc/occi-os,tmetsch/occi-os,enolfc/occi-os