Cleanly close connection when client closes connection prematurely

[larskuhtz]

The function Network.Socket.ByteString.recv returns a ByteString of length zero when the connection has been closed.
The mkConn function does not handle that special case and instead calls plainHTTP, which raises a spurious InsecureConnectionDemied Exception. On the client side http-client-tls (and probably most other clients) reacts to this with an ungraceful connection reset.

This PR adds a new ClientClosedConnectionPrematurely constructor to WarpTLSException. When the client closes a connection immediately after the opening handshake and before the TLS "hello" message, the connection is gracefully closed and a ClientClosedConnectionPrematurely exception is raised.

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[snoyberg]

@kazu-yamamoto this looks reasonable to me, but I think you'd be in a better position to review.

[kazu-yamamoto]

Looks excellent to me!

[kazu-yamamoto]

Merged. Thank you for your contribution!

[kazu-yamamoto]

A new version of warp-tls has been released.