Still fail but allow for testing this scenario.

tasks/main.yml

@@ -3,8 +3,10 @@
   fail:
       msg: "Currently Software Collections are only supported on CentOS 6. Do not install on other setups - see role README"
   when: ansible_distribution != "CentOS" or ansible_distribution_major_version != "6"
+  register: os_is_supported_check
 
 - name: Add Software Collections
   yum:
       name: centos-release-scl
       state: present
+  when: os_is_supported_check is succeeded