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.
Yeah - that was the source of my confusion about the `setdefault` thing. My original test case was wrong, but the same problem manifests if you use `ErrorDict()`.