Better handle case when IMAP THREAD extension not available

[kiil]

I am trying to use the envelope thread command, backend is gmail.

Example

himalaya envelope thread from Shila

But I am getting this error

The application panicked (crashed).
Message:  called `Result::unwrap()` on an `Err` value: ThreadMessagesError(ResolveTask(UnexpectedBadResponse(StatusBody { kind: Bad, code: None, text: Text("Unknown command: UID THREAD") })))
Location: /home/lk/.cargo/registry/src/index.crates.io-6f17d22bba15001f/email-lib-0.26.4/src/email/envelope/thread/imap.rs:67

Other commands, I have tried, work well.

[soywod]

Looks like Gmail does not support the IMAP THREAD extension, which is quite surprising. Let's keep this issue for the following purposes:

• get rid of the panic
• provide custom thread implementation in case the extension is not available