Replace panics with errors in thread spawning (#12040)

# Description
Replace panics with errors in thread spawning.

Also adds `IntoSpanned` trait for easily constructing `Spanned`, and an
implementation of `From<Spanned<std::io::Error>>` for `ShellError`,
which is used to provide context for the error wherever there was a span
conveniently available. In general this should make it more convenient
to do the right thing with `std::io::Error` and always add a span to it
when it's possible to do so.

# User-Facing Changes
Fewer panics!

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-command/src/network/http/client.rs

@@ -283,7 +283,7 @@ fn send_cancellable_request(
             let ret = request_fn();
             let _ = tx.send(ret); // may fail if the user has cancelled the operation
         })
-        .expect("Failed to create thread");
+        .map_err(ShellError::from)?;
 
     // ...and poll the channel for responses
     loop {