mail kit: Improve debugging of internal mail kit network code

* Return the correct error code if there is a connection error
* Be more verbose if there is a problem
  network connection problem instead of showing "Login error: "
* IMAP inbound: Show a valid error message if there is a
* Show server name resolution, fishes out IPv6 bug #8293

src/add-ons/mail_daemon/inbound_protocols/imap/IMAPInboundProtocol.cpp

@@ -345,8 +345,15 @@ IMAPInboundProtocol::Connect(const char* server, const char* username,
 	status_t status = fIMAPMailbox.Connect(server, username, password, useSSL,
 		port);
 	if (status != B_OK) {
+		if (fIMAPMailbox.CommandError().CountChars() > 0) {
+			// This was a IMAP error
 			statusMessage = "Failed to login: ";
 			statusMessage += fIMAPMailbox.CommandError();
+		} else {
+			// Probably a connection error
+			statusMessage = "Connection error: ";
+			statusMessage += strerror(status);
+		}
 		ShowError(statusMessage);
 		ResetProgress();
 		return status;

src/kits/mail/ServerConnection.cpp

@@ -224,14 +224,19 @@ SocketConnection::Connect(const char* server, uint32 port)
 	if (status != B_OK)
 		return status;
 
+	TRACE("Server resolves to %s\n", address.ToString().String());
+
 	fSocket = socket(address.Family(), SOCK_STREAM, 0);
-	if (fSocket < 0)
+	if (fSocket < 0) {
+		TRACE("%s: Socket Error: %s\n", __func__, strerror(status));
 		return errno;
+	}
 
 	int result = connect(fSocket, address, address.Length());
 	if (result < 0) {
+		TRACE("%s: Connect Error: %s\n", __func__, strerror(result));
 		close(fSocket);
-		return errno;
+		return result;
 	}
 
 	TRACE("SocketConnection: connected\n");