Fixed #22915 -- Document backward incompatible changes in the Validation...

...Error constructor.

This patch also fixes update_error_dict to better handle the use case described
in this ticket, previously the type of the provided container could be lost in
some conditions.
These three lines are essentially doing the fix - not the change to `update_error_dict`. Prior to these calls, printing `self._errors` shows that each "value" in self._errors is a `ValidationError()` instance; this converts the ValidationError instance into an ErrorList, which has the right behaviour when printed.