Binder: disambiguate unlinkToDeath error

BpBinder may return OK, DEAD_OBJECT, or NAME_NOT_FOUND for this
operation, but BBinder returns INVALID_OPERATION, and subclasses may
return other errors. In at least one bug, the resolution of this
ambiguity would help direct debugging.

Bug: 180891878
Test: N/A

Change-Id: I109a09788d3075e7c594445a6f3649ade109b5c2