Consistently use RAND_bytes and check for failure.

RAND_pseudo_bytes just calls RAND_bytes now and only returns 0 or 1. Switch all
callers within the library call the new one and use the simpler failure check.
This fixes a few error checks that no longer work (< 0) and some missing ones.

Change-Id: Id51c79deec80075949f73fa1fbd7b76aac5570c6
Reviewed-on: https://boringssl-review.googlesource.com/2621
Reviewed-by: Adam Langley <agl@google.com>