Give a login that reaches a KISS TNC its own exception type #246

Since 0.4.0-alpha.4 (7bb2078, #232), AprsIsClient fails a login straight away when a line contains FEND (0xC0), which means the other end is a KISS TNC, such as Dire Wolf's port 8001. It throws a plain LoginException. An app can only tell this apart from any other failed login by reading the English message.

AprsMap needs this to show "That's a KISS TNC: add it as a TNC source" instead of "Server unreachable" (its gap G32).

Scope: a LoginException subclass, for example KissTncLoginException. That follows LoginRejectedException, which is also a LoginException, so existing catch (LoginException) code keeps working. The failure should also reach AprsIsConnection's state events as that type. This adds to the public API only; behaviour is unchanged.

Done when:

• the AprsIsClientTests case for a login that reaches a KISS TNC checks the exception's type, not its message;
• the new type is listed in PublicAPI.Unshipped.txt.

AprsMap priority 3: nice to have for 0.5.0.