Make all fields of *gqlerror.Error public

[fredzqm]

I really like the validation.ErrorOption pattern. It makes it possible to build *gqlerror.Error with a chain of options.

gqlparser/validator/validator.go

Lines 40 to 48 in 8d1bedc

	rule.rule(observers, func(options ...ErrorOption) {
	err := &gqlerror.Error{
	Rule: rule.name,
	}
	for _, o := range options {
	o(err)
	}
	errs = append(errs, err)
	})

However, it cannot set err in the accumulating *gqlerror.Error. Why can't all fields of *gqlerror.Error be public?

I have:

• Added tests covering the bug / feature
• Updated any relevant documentation

[StevenACoffman]

Yeah, not sure why it wasn't public before. Thanks!