Provide clearer error when deleting enabled domain

Currently the message is confusing, because it reads like an instruction
to delete an enabled domain, rather than an instruction that you can
only delete disabled domains.  Hopefully this wording is a little clearer
for users.

Change-Id: I5189cd1bbae0e485d03660be19f20879110a3b3e
Closes-Bug: #1279823
Merge "Provide clearer error when deleting enabled domain"