Panic when using WrapError(), Errorf() or SourceFile() in a block

[wttw]

Checklist

• [X] I have searched the issue list
• [ ] I have tested my example against Shopify Liquid. (This isn't necessary if the actual behavior is a panic, or an error for which IsTemplateError returns false.)

Detailed Description

If a custom block, created with RegisterBlock(), uses render.Context.WrapError it panics due to render.rendererContext.node being nil.

Errorf() and SourceFile() have the same issue.

Test case:

package main

import (
    "errors"
    "github.com/osteele/liquid"
    "github.com/osteele/liquid/render"
)

func main() {
    engine := liquid.NewEngine()
    engine.RegisterBlock("foo", func(c render.Context) (string, error) {
        return "bar", c.WrapError(errors.New("baz"))
    })
    _, _ = engine.ParseAndRenderString(`{% foo %}  {% endfoo %}`, map[string]interface{}{})

}

Possible Solution