tls: better error message for socket disconnect

The error emitted when a connection is closed before the TLS handshake completes seemed rather unspefic by just saying socket hang up.

Use a more verbose message, that also indicates that this is a purely client-side error, and remove a misleading comment.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

tls