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.
Gah - Sorry - I'm confusing things here.
* My original test case passed in `ErrorList` when it should have passed in `ErrorDict`
* In the original version of the 1.7 code, it didn't matter if you used `ErrorList` or `ErrorDict` - you saw the same result (an `ErrorDict` filled with ValidationError instances)
* As a result of this patch, it *does* matter which one you use (`ErrorList` breaks with an error about not having `setdefault`, `ErrorDict` works) - however, on output, you still get an `ErrorDict` filled with ValidationError instances.

In summary - my very first analysis (not the comment on this particular line - my overall comment about how the patch was broken with the test case provided in the patch) was incorrect.