Map DirectoryNotFound to FileNotFound for open command (#10089)

# Description


This PR should close #10085
Maps `DirectoryNotFound` errors to `FileNotFound`. All other errors are
left unchanged.

# User-Facing Changes

This means a user will see `FileNotFound` instead of `DirectoryNotFound`
which is more meaning full to the user.

crates/nu-command/tests/commands/open.rs

@@ -169,7 +169,7 @@ fn errors_if_file_not_found() {
     //
     // This seems to be not directly affected by localization compared to the OS
     // provided error message
-    let expected = "not found";
+    let expected = "File not found";
 
     assert!(
         actual.err.contains(expected),