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

```
> ^echo (char -i 0)
Error: nu::shell::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::shell::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 nushell#1:1:2]
 1 │ ^echo (char -i 0)
   ·  ──┬─
   ·    ╰── Invalid input parameter
   ╰────
```

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