libbinder: RpcState stop narrowing errors

We want better errors (luckily DEAD_OBJECT is -EPIPE. The way libbinder
uses status_t is very compatible with the errnos returned from the
system calls we're using). Whether this is a good design remains to be
seen.

Bug: N/A
Test: binderRpcTest
Change-Id: Ice446ec48307d697334538687676f806c08289a1