Add GetUnderlying function to ErrorWithPos

[bufdev]

I need access to the underlying error without any source information, this exposes it. Everything works if you just add the function, I added specific tests cases as well though.

This effectively makes ErrorWithPos equal to the old ErrorWithSourcePos in terms of what it exposes, but as an interface.

[tie]

Hi! That’s a nice feature, but you should take a look at the current developments in Go. The next release (Go 1.13) will have errors.Unwrap function that unwraps the underlying error if it the wrapper implements interface { Unwrap() error }.

[jhump]

Good catch.

[bufdev]

That's fine @tie, but protoreflect is meant to be compatible with previous Golang versions, thanks for checking this over though, always good to have new people interested in this amazing library by @jhump.

[jhump]

Since this PR isn't actually released, it may be nice to rename the method to Unwrap. As @tie pointed out, that will allow it to transparently work with errors.Unwrap for people that are using Go 1.13.

[tie]

@pedgeio, simply renaming GetUnderlying to Unwrap would make it forward-compatible too.

[bufdev]

Yea I'm down with that, want me to put up a PR or can you push it quickly?

[jhump]

Do you mind? I actually have some other changes I'm working on at the moment and don't want to get side-tracked. If you don't have a chance, I can get to it later today.

[bufdev]

Yep I'll do it right now

[bufdev]

#261