Better error on spawn failure caused by null bytes (#15911)

# Description

When attempting to pass a null byte in a commandline argument, Nu
currently fails with:

```
> ^echo (char -i 0)
Error: nu:🐚:io::invalid_input

  × I/O error
  ╰─▶   × Could not spawn foreground child

   ╭────
 1 │ crates/nu-command/src/system/run_external.rs:284:17
   · ─────────────────────────┬─────────────────────────
   ·                          ╰── Invalid input parameter
   ╰────
```

This does not explain which input parameter is invalid, or why. Since Nu
does not typically seem to escape null bytes when printing values
containing them, this can make it a bit tricky to track down the
problem.

After this change, it fails with:

```
> ^echo (char -i 0)
Error: nu:🐚:io::invalid_input

  × I/O error
  ╰─▶   × Could not spawn foreground child: nul byte found in provided data

   ╭────
 1 │ crates/nu-command/src/system/run_external.rs:282:17
   · ─────────────────────────┬─────────────────────────
   ·                          ╰── Invalid input parameter
   ╰────

```

which is more useful. This could be improved further but this is niche
enough that is probably not necessary.

This might make some other errors unnecessarily verbose but seems like
the better default. I did check that attempting to execute a
non-executable file still has a reasonable error: the error message for
that failure is not affected by this change.

It is still an "internal" error (referencing the Nu code triggering it,
not the user's input) because the `call.head` span available to this
code is for the command, not its arguments. Using it would result in

```
  × I/O error
  ╰─▶   × Could not spawn foreground child: nul byte found in provided data

   ╭─[entry #1:1:2]
 1 │ ^echo (char -i 0)
   ·  ──┬─
   ·    ╰── Invalid input parameter
   ╰────
```

which is actively misleading because "echo" does not contain the nul
byte.

# User-Facing Changes
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# Tests + Formatting
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
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Haven't tried to write a test yet: it's tricky because the better error
message comes from the Rust stdlib (so a straightforward integration
test checking for the specific message would be brittle)...

# After Submitting
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crates/nu-command/src/system/run_external.rs

@@ -275,11 +275,8 @@ impl Command for External {
         );
 
         let mut child = child.map_err(|err| {
-            IoError::new_internal(
+            let context = format!("Could not spawn foreground child: {err}");
-                err,
+            IoError::new_internal(err, context, nu_protocol::location!())
-                "Could not spawn foreground child",
-                nu_protocol::location!(),
-            )
         })?;
 
         if let Some(thread_job) = engine_state.current_thread_job() {