ceph-backport.sh/interactive_setup: abort if Step 3 fails

ceph-backport.sh/interactive_setup: abort if Step 3 fails

Fixes confusing error message reported by a user:

Setup step 3 of 4 - remote repos
---------------------------------------------------------------------
Searching "git remote -v" for remote repos

ceph-backport.sh: ERROR: Cannot auto-determine fork remote
ceph-backport.sh: (Could not find GitHub user callithea's fork of ceph/ceph in
"git remote -v")
ceph-backport.sh: ERROR: (internal error) fork_remote not set, even after
completing Steps 1-3 of interactive setup
ceph-backport.sh: This could be reported as a bug!

Signed-off-by: Nathan Cutler <ncutler@suse.com>