QUIC: adjusted handling of callback errors.

QUIC: adjusted handling of callback errors.

Changed handshake callbacks to always return success.  This allows to avoid
logging SSL_do_handshake() errors with empty or cryptic "internal error"
OpenSSL error messages at the inappropriate "crit" log level.

Further, connections with failed callbacks are closed now right away when
using OpenSSL compat layer.  This change supersedes and reverts c37fdcdd1,
with the conditions to check callbacks invocation kept to slightly improve
code readability of control flow; they are optimized out in the resulting
assembly code.