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/system/run_external.rs

@@ -2,6 +2,7 @@ use nu_cmd_base::hook::eval_hook;
 use nu_engine::env_to_strings;
 use nu_engine::eval_expression;
 use nu_engine::CallExt;
+use nu_protocol::IntoSpanned;
 use nu_protocol::NuGlob;
 use nu_protocol::{
     ast::{Call, Expr},
@@ -438,7 +439,7 @@ impl ExternalCommand {
 
                                 Ok(())
                             })
-                            .expect("Failed to create thread");
+                            .map_err(|e| e.into_spanned(head))?;
                     }
                 }
 
@@ -526,7 +527,7 @@ impl ExternalCommand {
                             Ok(())
                         }
                     }
-                }).expect("Failed to create thread");
+                }).map_err(|e| e.into_spanned(head))?;
 
                 let (stderr_tx, stderr_rx) = mpsc::sync_channel(OUTPUT_BUFFERS_IN_FLIGHT);
                 if redirect_stderr {
@@ -543,7 +544,7 @@ impl ExternalCommand {
                             read_and_redirect_message(stderr, stderr_tx, stderr_ctrlc);
                             Ok::<(), ShellError>(())
                         })
-                        .expect("Failed to create thread");
+                        .map_err(|e| e.into_spanned(head))?;
                 }
 
                 let stdout_receiver = ChannelReceiver::new(stdout_rx);