Handle attempt to subscribe a local user via their remote webfinger reference more...

Handle attempt to subscribe a local user via their remote webfinger reference more gracefully.
The ensure* family of functions will now return an OStatusShadowException in this case, which gives us a pleasant error message instead of a giant exception backtrace when you do 'sub somebody@this.local.server'.

Can be extended later to allow actually using the local profile, since we could figure it out.