Fix mv error message issues (arrows, Windows paths) (#7197)

# Description

BEFORE (notice Windows paths look wrong):
```
〉mv 8 9
Error:
  × Destination file already exists
   ╭─[entry #22:1:1]
 1 │ mv 8 9
   ·      ┬
   ·      ╰── you can use -f, --force to force overwriting the destination
   ╰────

〉mv d1 tmp
Error:
  × Can't move "C:\\Users\\Leon\\TODO\\d1" to "C:\\Users\\Leon\\TODO\\tmp\\d1"
   ╭─[entry #19:1:1]
 1 │ mv d1 tmp
   ·       ─┬─
   ·        ╰── Directory not empty
   ╰────

```
AFTER (full paths are now included in the arrows' messages to make lines
like `mv $foo` entirely unambiguous):
```
〉mv 8 9
Error:
  × Destination file already exists
   ╭─[entry #4:1:1]
 1 │ mv 8 9
   ·      ┬
   ·      ╰── Destination file 'C:\Users\Leon\TODO\tmp\9' already exists
   ╰────
  help: you can use -f, --force to force overwriting the destination

〉mv d1 tmp
Error:
  × Can't move 'C:\Users\Leon\TODO\d1' to 'C:\Users\Leon\TODO\tmp\d1'
   ╭─[entry #3:1:1]
 1 │ mv d1 tmp
   ·       ─┬─
   ·        ╰── Directory 'C:\Users\Leon\TODO\tmp' is not empty
   ╰────

```
# User-Facing Changes

See above.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace --features=extra -- -D warnings -D
clippy::unwrap_used -A clippy::needless_collect` to check that you're
using the standard code style
- `cargo test --workspace --features=extra` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/filesystem/mv.rs

@@ -107,9 +107,15 @@ impl Command for Mv {
         if destination.exists() && !force && !destination.is_dir() && !source.is_dir() {
             return Err(ShellError::GenericError(
                 "Destination file already exists".into(),
-                "you can use -f, --force to force overwriting the destination".into(),
+                // These messages all use to_string_lossy() because
+                // showing the full path reduces misinterpretation of the message.
+                // Also, this is preferable to {:?} because that renders Windows paths incorrectly.
+                format!(
+                    "Destination file '{}' already exists",
+                    destination.to_string_lossy()
+                ),
                 Some(spanned_destination.span),
-                None,
+                Some("you can use -f, --force to force overwriting the destination".into()),
                 Vec::new(),
             ));
         }
@@ -119,20 +125,26 @@ impl Command for Mv {
         {
             return Err(ShellError::GenericError(
                 "Can only move multiple sources if destination is a directory".into(),
-                "destination must be a directory when multiple sources".into(),
+                "destination must be a directory when moving multiple sources".into(),
                 Some(spanned_destination.span),
                 None,
                 Vec::new(),
             ));
         }
 
+        // This is the case where you move a directory A to the interior of directory B, but directory B
+        // already has a non-empty directory named A.
         if source.is_dir() && destination.is_dir() {
             if let Some(name) = source.file_name() {
                 let dst = destination.join(name);
                 if dst.is_dir() {
                     return Err(ShellError::GenericError(
-                        format!("Can't move {:?} to {:?}", source, dst),
+                        format!(
-                        "Directory not empty".into(),
+                            "Can't move '{}' to '{}'",
+                            source.to_string_lossy(),
+                            dst.to_string_lossy()
+                        ),
+                        format!("Directory '{}' is not empty", destination.to_string_lossy()),
                         Some(spanned_destination.span),
                         None,
                         Vec::new(),
@@ -148,8 +160,8 @@ impl Command for Mv {
             if let Some(Ok(filename)) = some_if_source_is_destination {
                 return Err(ShellError::GenericError(
                     format!(
-                        "Not possible to move {:?} to itself",
+                        "Not possible to move '{}' to itself",
-                        filename.file_name().unwrap_or(filename.as_os_str())
+                        filename.to_string_lossy()
                     ),
                     "cannot move to itself".into(),
                     Some(spanned_destination.span),

crates/nu-command/tests/commands/move_/mv.rs

@@ -403,7 +403,7 @@ fn mv_directory_with_same_name() {
             "#
         );
 
-        assert!(actual.err.contains("Directory not empty"));
+        assert!(actual.err.contains("is not empty"));
     })
 }