To replicate:
* Create a package group (test-group-2)
* Create a category (test-category-1)
* Add test-group-2 to test-category-1
* Delete test-group-2

The category in comps.xml still references test-group-2. We need to either:
* Have Pulp remove it from the category
* Not let the user delete the group until it's explicitly removed from the category

The easiest way to see this is to open a second terminal and do a watch on the comps.xml file in the repo:

watch -n 1 cat comps.xml

After deleting the group, my comps.xml was left in an inconsistent state:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE comps PUBLIC "-//Red Hat, Inc.//DTD Comps info//EN" "comps.dtd">
<comps>

  <group>
   <id>test-group-1</id>
   <default>true</default>
   <uservisible>true</uservisible>
   <display_order>1024</display_order>
   <name>Test Group 1</name>
   <description>Test Group 1 Description</description>
    <packagelist>
      <packagereq type="default">gofer</packagereq>
    </packagelist>
  </group>
  <category>
   <id>test-category-1</id>
   <display_order>99</display_order>
   <name>Test Category 1</name>
   <description>Test Category 1 Description</description>
    <grouplist>
     <groupid>test-group-2</groupid>
    </grouplist>
  </category>

</comps>