Make the panel_view agnostic to error extensions

`LintError` is an open type and not all types are hashable.  Make
`format_error` in `panel_view` resilient and only pass down items
actually needed for drawing.

This really only circumvents a limitation of python's `lru_cache`
implementation that relies on hashable types instead of simple
equality.

panel_view.py

@@ -423,7 +423,7 @@ def format_error(error, widths):
     error_as_tuple = tuple(
         (k, v)
         for k, v in error.items()
-        if k != 'region'  # region is not hashable
+        if k in ("line", "start", "error_type", "linter", "msg", "code", )
     )
     return _format_error(error_as_tuple, widths)