appliance_console#fetch_key fix "try again"

When the appliance fails to download a key, ask try again.

Old code did not properly detect the failure
(also fixed prompting message to be more clear)

https://bugzilla.redhat.com/show_bug.cgi?id=1145365
I think lowercase is right. because there is no default.
But you are right about consistency, so I fixed. thanks