add "default" table theme (#11072)

# Description

This PR fixes a minor bug that prevented this command from running.
```nushell
table --list | each {|r| print ($r); print (ls | first 3 | table --theme $r)}
```
Here's the output now of the first few themes.

![image](https://github.com/nushell/nushell/assets/343840/21bc8942-5106-4b6a-8905-e90d6cb9a153)

It prevented it from running because "default" wasn't a real table
theme. Now "default" is a synonym of rounded.

Also tweaked the error message when a bad theme name is provided.

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crates/nu-command/src/viewers/table.rs

@@ -320,14 +320,20 @@ fn get_theme_flag(
     state: &EngineState,
     stack: &mut Stack,
 ) -> Result<Option<TableMode>, ShellError> {
-    call.get_flag(state, stack, "theme")?
+    let theme_name = call.get_flag(state, stack, "theme")?;
+    theme_name
+        .clone()
         .map(|theme: String| TableMode::from_str(&theme))
         .transpose()
         .map_err(|err| ShellError::CantConvert {
             to_type: String::from("theme"),
             from_type: String::from("string"),
             span: call.span(),
-            help: Some(String::from(err)),
+            help: Some(format!(
+                "{}, but found '{}'.",
+                String::from(err),
+                theme_name.unwrap_or("".to_string())
+            )),
         })
 }