libbinder_ndk: more descriptive prune error

We collapse errors into UNKNOWN_ERROR in the NDK API in order to avoid
exposing certain internal details. Now, the log when this happens
includes more information (instead of the code only).

Bug: N/A
Test: see:
    09-22 17:09:29.322  5753  5770 W roid.binder.ct: PruneStatusT:
    Unknown status_t (-129 (Key was rejected by service)) pruned into
    STATUS_UNKNOWN_ERROR
Change-Id: I4f47574857c47341b9500981101adcae75e864c7