Description of problem:
If you use an repo-id that is not bound to the consumer group, the error message displayed is wrong.

Version-Release number of selected component (if applicable):
[root@hp-dl380pgen8-02-vm-2 ~]# rpm -qa pulp-server
pulp-server-2.5.0-0.19.rc.el7.noarch


How reproducible:


Steps to Reproduce:
1.create a consumer group 
2.add consumers to the group
3.try to unbind a repo that is not bound to the consumer group

Actual results:

[root@hp-dl380pgen8-02-vm-2 ~]# rpm -qa pulp-server
pulp-server-2.5.0-0.19.rc.el7.noarch
[root@hp-dl380pgen8-02-vm-2 ~]# 
[root@hp-dl380pgen8-02-vm-2 ~]# 
[root@hp-dl380pgen8-02-vm-2 ~]# pulp-admin rpm consumer group members list --group-id grp1
+----------------------------------------------------------------------+
                         Consumer Group Members
+----------------------------------------------------------------------+

Id:           client1
Display Name: client1
Description:  None
Notes:        


[root@hp-dl380pgen8-02-vm-2 ~]# pulp-admin rpm consumer group list
+----------------------------------------------------------------------+
                            Consumer Groups
+----------------------------------------------------------------------+

Id:           grp1
Display Name: None
Description:  None
Consumer Ids: client1
Notes:        



[root@hp-dl380pgen8-02-vm-2 ~]# pulp-admin rpm consumer group unbind --consumer-group-id grp1 --repo-id rhel7-client
Consumer Group [grp1] does not exist on the server

Expected results:


Additional info: