tls_socket: fix handling of syscall errors
tls_socket: fix handling of syscall errors

This prevents a case where we'd see an error like 'TlsSocket::Recv: Success'
when trying to read from a TLS socket which got abruptly shut down by the
remote side.

Change-Id: I9a0a63f861d71bd3186567ff98148476795530ab
Reviewed-on: http://gerrit.cloudera.org:8080/5954
Tested-by: Kudu Jenkins
Reviewed-by: Dan Burkert <4ef91e292a55314d2653d35ff669a6bd939ce515@apache.org>

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