Switch to Go 1.13 error types

[cyphar]

So we don't need to use github.com/pkg/errors anymore. Unfortunately this'll mean we can't get stack traces anymore but I don't think that'll be the end of the world.

[tych0]

I dunno, I kind of like the stack traces :)

[cyphar]

Yeah I do too, the minor issue is that it's not clear to me whether errors.Is can reach all the way through both fmt.Errorf("%w") wrapping as well as errors.Wrap wrapping. On paper it should be able to, since pkg/errors got support for Go 1.13 errors but I'm not entirely sure.

The main thing is that we need to switch away from errors.Cause because as more libraries switch to Go 1.13 errors it'll become more and more brittle.

[danishprakash]

@cyphar do we have a consensus here? I'd like to work on this.