plan, executor: check b.err after buildSort and buildLimit in builUnion

[XuHuaiyu]

What have you changed? (mandatory)

Before this commit, b.err is not checked after buildSort and buildLimit in buildUnion.

What is the type of the changes? (mandatory)

• Bug fix (non-breaking change which fixes an issue)

How has this PR been tested? (mandatory)

integration test

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

no

Does this PR need to be added to the release notes? (mandatory)

no

Refer to a related PR or issue link (optional)

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[shenli]

LGTM

[shenli]

/run-all-tests

[lysu]

I have an idea how about extract this part to this style.

package main

import (
	"github.com/juju/errors"
	"fmt"
)

type builder struct {
	err error
}

func (b *builder) b1() {
	b.b2()
	b.traceSelfCallStack()
	return
}

func (b *builder) b2() {
	b.err = errors.New("111")
	return
}

func (b *builder) traceSelfCallStack() {
	if b.err == nil {
		return
	}
	err := errors.NewErrWithCause(b.err, "")
	err.SetLocation(2)
	b.err = &err
	return
}

and still, keep every builder self-call has a b.traceSelfCallStack() followed..(we have no generic, so we can not b.selfCall(b.b2)() which is more readable)

[lysu]

LGTM

[lysu]

/run-integration-ddl-test