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-table/src/common.rs

@@ -182,6 +182,7 @@ pub fn load_theme(mode: TableMode) -> TableTheme {
         TableMode::Compact => TableTheme::compact(),
         TableMode::WithLove => TableTheme::with_love(),
         TableMode::CompactDouble => TableTheme::compact_double(),
+        TableMode::Default => TableTheme::rounded(),
         TableMode::Rounded => TableTheme::rounded(),
         TableMode::Reinforced => TableTheme::reinforced(),
         TableMode::Heavy => TableTheme::heavy(),