FIX: have consistent errors between std help and std help ... (#9101)

# Description
an example should show what happens quite clearly 😋 

> **Note**
> the ugly spanned errors you'll see below are fixed in
https://github.com/nushell/nushell/pull/9039 😌

in all cases we get
```
> std help commands does-not-exist-anywhere
Help pages from external command does-not-exist-anywhere:
No manual entry for does-not-exist-anywhere
Error:
  × std::help::command_not_found
     ╭─[help:662:1]
 662 │
 663 │     let command = ($command | str join " ")
     ·     ─┬─
     ·      ╰── command not found
 664 │
     ╰────
```
but

## ❌ before this PR
```
> std help does-not-exist-anywhere
Help pages from external command does-not-exist-anywhere:
No manual entry for does-not-exist-anywhere
```
without any error, which makes it inconsistent with all the other `std
help` commands which give errors when not finding an item 🤔

## ✔️ with this PR
```
> std help does-not-exist-anywhere
Help pages from external command does-not-exist-anywhere:
No manual entry for does-not-exist-anywhere
Error:
  × std::help::item_not_found
     ╭─[help:740:1]
 740 │
 741 │     let item = ($item | str join " ")
     ·     ─┬─
     ·      ╰── item not found
 742 │
     ╰────
```

# User-Facing Changes
more consistent errors when using `std help` and `std help commands`, as
shown above.

# Tests + Formatting
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# After Submitting
```
$nothing
```

crates/nu-std/lib/help.nu

@@ -718,4 +718,14 @@ You can also learn more at (ansi default_italic)(ansi light_cyan_underline)https
 
     let modules = (try { help modules $item --find $find })
     if not ($modules | is-empty) { return $modules }
+
+    let span = (metadata $item | get span)
+    error make {
+        msg: ("std::help::item_not_found"  | error-fmt)
+        label: {
+            text: "item not found"
+            start: $span.start
+            end: $span.end
+        }
+    }
 }