Fix badly encoded paths with an exception view instead

[seanh]

Pyramid actually raises a specific URLDecodeError when there's a badly-encoded path component, but by default it 500s because there isn't a builtin exception view for URLDecodeError. Apps or extensions can register a custom exception view to catch this and turn it into a 400:

@view_config(context=URLDecodeError)
def url_decode_error(request):
    return HTTPBadRequest()

This is simpler than our current invalid_path_info_tween_factory() and is also the right solution according to the Pyramid docs for URLDecodeError and multiple GitHub comments from Pyramid maintainers.

Unfortunately Pyramid does not raise URLDecodeError for badly encoded query params, so that one will still have to be fixed by a tween.

See Pylons/pyramid#312 (comment) (and there are also several other maintainer comments on GitHub telling people to use an exception view for this)