Update pack register API endpoint to throw a more user-friendly exception in case requested pack doesn't exist.
Update pack register API endpoint to throw a more user-friendly exception in
case requested pack doesn't exist.

StackStorm/st2,StackStorm/st2,nzlosh/st2,peak6/st2,lakshmi-kannan/st2,Plexxi/st2,Plexxi/st2,Plexxi/st2,tonybaloney/st2,peak6/st2,lakshmi-kannan/st2,nzlosh/st2,lakshmi-kannan/st2,StackStorm/st2,tonybaloney/st2,tonybaloney/st2,peak6/st2,StackStorm/st2,nzlosh/st2,Plexxi/st2,nzlosh/st2