Return UnsuportedCommand (INVALID_COMMAND) instead of NotAvailable (INVALID_LENGTH)

This allows nitropy to easily skip the test if the command is not supported

src/admin.rs

@@ -276,7 +276,7 @@ where
                 }
                 #[cfg(not(feature = "se050"))]
                 {
-                    return Err(Error::NotAvailable);
+                    return Err(Error::UnsupportedCommand);
                 }
             }
             Command::GetConfig => {