Fixes #1890

Rename `ca1-cert.pem` to `ca-certificate.pem`, `ca1-key.pem` to
`ca-key.pem`, `agent1-cert.pem` to `client-certificate.pem`, and
`agent1-key.pem` to `client-key.pem`.

Instead of destroying the socket, try to close the connection cleanly
if an error (such as a data framing error) occurs after the opening
handshake has completed.

Also, to comply with the specification, use the 1006 status code if no
close frame is received, even if the connection is closed due to an
error.

Fixes #1898

Call `ws.terminate()` only if `duplex.destroy()` is called directly by
the user and not indirectly by the listener of the `'error'` event of
the `WebSocket` object. Calling `ws.terminate()` right after the
`'error'` event is emitted on the `WebSocket` object, might prevent the
close frame from being sent to the other peer.

Fixes #1892