rest-api: fix error response in LogTailViewSet

Signed-off-by: Gregory Meno <gregory.meno@inktank.com>

I broke this in a bad-merge 63e574d59
travis: add --devmode to calamari-ctl initialize

Signed-off-by: Gregory Meno <gregory.meno@inktank.com>
Merge pull request #170 from ceph/wip-fix-master2

Wip fix master2