Alter SignedExtension and related to cater for more advanced exts like SkipCheckIfFeeless

[jsdw]

SkipCheckIfFeeless is a signed ext that wraps another, but externally just has the name SkipCheckIfFeeless.

The current impl of it on master will return an error if the ext is not present on a chain, rather than being able to be silently ignored if not needed. But more than that, matching on just the name of the signed ext is bound to lead to future issues with this sort of ext in existence (if a chain has two exts wrapped in SkipCheckIfFeeless, we won't be able to match things up properly any more).

So, the goal here will be to modify the related traits so that signed extensions can search for themselves more intelligently, and also have the ability to return errors at the point of encoding (because they become more possible.