Return bool from C++ functions in bssl_shim.

Also move BIO_print_errors_fp up a level so it's less repetitive. There's
enough exit points now that it doesn't seem like adding a separate return exit
code for each has held up. (Maybe there should be a macro that samples
__LINE__...)

Change-Id: I120e59caaa96185e80cf51ea801a5e1f149b1b39
Reviewed-on: https://boringssl-review.googlesource.com/3530
Reviewed-by: Adam Langley <agl@google.com>