More consistent way of handling validation errors

[oxisto]

This PR removes the mess of the old ValidationError struct and replaces it with either multiple wrapped errors (in case of Go 1.20) or an implementation that emulates that behaviour using internal structs (on Go < 1.20).

It also consistently aborts the validation process now if signature validation fails, which otherwise might lead to unexpected error situations.

[mfridman]

This is fantastic work!

The new error handling reads a lot better in my opinion and hopefully, in a few releases, we can drop the older error branch and build tags entirely.