libbinder_ndk: clarify error prune functions

libbinder_ndk has some functionality to prevent unknown errors from
propagating out of an NDK API, but these error messages didn't clearly
indicate that the number they were printing is the unknown error value,
not the value the error is changed to.

Bug: 166822339
Test: N/A
Change-Id: I54a8aa22413cb5b32f2a75e84ddaeb05fc378b9e