Unhandled alert

[reynir]

While testing https://github.com/hannesm/ocaml-tls/tree/no-cstruct I discovered that openssl s_client sends an alert in a place where we don't handle it:

ocaml-tls/lib/engine.ml

Line 309 in fc78dd8

| _ -> Error (`Fatal `InvalidMessage))

The alert in question is fatal bad record mac. I attempted the following to get better error reporting, but I suspect it is not quite right.

     | Packet.ALERT ->
       let* alert = map_reader_error (Reader.parse_alert buf) in
       (match alert with
        | _, BAD_RECORD_MAC -> Error (`Fatal `MACMismatch)
        | _, _ -> Error (`Fatal `InvalidMessage))

[hannesm]

Thanks for your report, I added a nicer error message in e4e9c95