Add error name for dict

The idea behind this PR is for HTTP views. Let's say we want to validate the user input that is passed to an HTTP endpoint through url parameters, JSON or form data. This is usually under the form of a `dict`. Thanks to this PR, we can validate those data, catch the `SchemaError` exception and get the key that was invalid, as well as its error content. This is useful to display error next to a form field for instance.