[rights] Add some status checking to devmgr

In refactoring rights, I broke a bunch of these, but things mostly keep
working quietly in a not-quite-right way, because launchpad happily
takes ZX_HANDLE_INVALID and fires new things into the default root job,
rather than where they were supposed to be going.

No intended change during normal behavior.

ZX-2967 #comment [rights] Add some status checking to devmgr

Test: CQ
Change-Id: Ic0226e5c128665e425f137bb0472250d2fba6048