Better error feedback

[amaury1093]

When there's an error on the SMTP step, we return a Err(SmtpError::<some error>). There are cases where the error is not descriptive enough: io: incomplete, future timed out...

Proposal 1: Add the STMP step in the response

{
  // snip
  "smtp": {
    "error": {
      "type": "SomeError",
      "message": "future timed out",
+     "step": "connect" // or "helo", "mail_from", "rcpt_to", "close", to know on which step it's timing out
    }
  }
}

Proposal 2: Add all logs in the reponse (optional)

When checking an email, we can add an option to include logs in the response.

{
  // snip
  "smtp": {
    "error": {
      "type": "SomeError",
      "message": "future timed out",
    }
  },
+ "logs": [
+    "line1..."
+    // snip
+ ]
}

[Nyceane]

what is io: incomplete?