A set of fixes for stderr redirect (#7219)

# Description

This is a set of fixes to `err>` to make it work a bit more predictably.

I've also revised the tests, which accidentally tested the wrong thing
for redirection, but should be more correct now.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/core_commands/do_.rs

@@ -168,6 +168,9 @@ impl Command for Do {
                 metadata,
                 trim_end_newline,
             }),
+            Ok(PipelineData::Value(Value::Error { .. }, ..)) if ignore_shell_errors => {
+                Ok(PipelineData::new(call.head))
+            }
             Err(_) if ignore_shell_errors => Ok(PipelineData::new(call.head)),
             r => r,
         }

crates/nu-command/src/system/run_external.rs

@@ -390,7 +390,7 @@ impl ExternalCommand {
                                 use std::os::unix::process::ExitStatusExt;
                                 if x.core_dumped() {
                                     let style = Style::new().bold().on(Color::Red);
-                                    println!(
+                                    eprintln!(
                                         "{}",
                                         style.paint(format!(
                                             "nushell: oops, process '{commandname}' core dumped"