Check result of ThousandIsland.Socket.handshake/1

[moogle19]

Added a check for the ThousandIsland.Socket.handshake/1 return value, in case it fails (e.g. if the path to the tls cert is messed up).

PS: Sorry for the many changes, but my editor removed the whitespaces at the end of the lines.
The real change is just the case statement at the bottom.

[mtrudel]

Great PR! I can't believe I missed that in my original workup.

A couple of requests in advance of merging:

1. Would you be able to clean up the extra whitespace, just to keep the PR tight? (feel free to rebase / push a revert commit as desired; I'll squash this branch down when merging)
2. Could you please cover this in a test (you can induce a busted handshake pretty easily with tls options on the client - likely by mandating a super small set of ciphers or having the server require a client cert).

[ewildgoose]

I also think that the typespec for Socket.handshake is incorrect? It specifies something like {:error, String.t()}, however, looking at the erlang docs for handshake() it defines the return value as :ssl.reason(), which is in turn defined as:

closed | timeout | error_alert()

I'm wondering if we should use :ssl.reason() or any(), since arguably it's overrideable by a custom transport? Opinion?

[mtrudel]

The SSL module has some odd bits in its typespecs, and if memory serves I think I took a bit of a shortcut there and just punted on the error tuple spec. If you want to sort them all out and tighten up our public spec @ewildgoose that would be very welcome work. So long as dialyzer passes I'm happy.

[moogle19]

@mtrudel
I added some tests, but I am not really happy with using capture_log for it.
But I am not quite sure how to test a failure in a single handler without some hacky stuff like capture_log or (ab)use the TelemetryCollector to collect the [:handler, :error].

Do you have any other ideas how to extract the error value from a single failing handler?

[moogle19]

Okay, I think I found an acceptable solution by sending the error result to the test process and checking it via assert_received.
If you have a better solution, please let me know!

[mtrudel]

Solution looks good! Just waiting on CI.