Form-level errors

[azmeuk]

Fixes #55

Adds a Form.form_errors list attribute to store form level errors. The content of form_errors is available in Form.errors[None].

I have no strong ideas about Form.errors[None], or Form.errors["__all__"], even if I prefer a keyword such as None for such a special key. __all__ is fine too if you prefer.

[davidism]

I like None more.

[oTree-org]

Hi, I am using wtforms in my project (thank you by the way) and just wanted to comment on this, even though it may be too late. I actually think '__all__' is a nice name because:

• It is a string, like all the other keys in the dict. This gives the dict a simpler type signature.
• '__all__' is something recognizable and searchable. I can search my codebase and the WTForms docs for '__all__' to get more info about it and where it is used, but None is hard to search for. Some people might see None and not know what it means.
• This is what Django uses, so many people know it by convention.

Just my 2 cents but I don't have a strong opinion.