Return BoringSSL error messages from multithreaded encryption processing

The SSLFilter::ProcessFilterRequest function is called from the IO thread pool, on a different thread than the Dart isolate.  Make SSLFIlter::ProcessAllBuffers return an integer status value from the BoringSSL calls, and if it is an error status, read info from the BoringSSL error stack, and pass it back through the C function interface to the caller, and report it to Dart.
