tls: handle `error` events with `_tlsError`

Previously `TLSSocket#_emitTLSError` was used as an `error` event
handler. However that function can emit `error` event itself, so it is
not suitable for such use. Luckily the event can be emitted only when
the control is released, so this looping-error can't happen.

Replace the error handler for clarity and correctness.

PR-URL: https://github.com/nodejs/node/pull/8889


Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>