Support for form-level error

[phuctm97]

Currently, Mantine Form only supports field-level errors, it would be great to support form-level error as well since Mantine is already wrapping onSubmit, Mantine can easily catch submit errors and set to a form-level error. Currently, I need to try catch in every onSubmit and set form-level error myself, which works but feels unintuitive.

Furthermore, Mantine Form can support server/submission-level error handling by having an option in useForm, something like tranformError, which will take the error thrown during onSubmit and return Mantine Form errors (field errors and form-level errors).