plugin/file should not reload with wrong file

I use plugin/auto as the main plugin to provide dns service. Zone files are dynamically generated by other programs, and plugin/auto will automatically reload these files regularly. These zone files may have errors, for example an A record is set to 127.0.0.300 like the following:
```
@ 600 IN SOA ns1.my.com. dns.my.com. 1716348976 900 180 1209600 180
@ 600 IN NS ns1.my.com.
right 600 IN A 127.0.0.1
wrong 600 IN A 127.0.0.300
right2 600 IN A 127.0.0.2
```

The current implementation will ignore the error line and the lines after it, saying "wrong" and "right2", and make the lines before the error line valid, saying "SOA", "NS" and "right". This causes some data to be lost and errors to be hidden.

https://github.com/coredns/coredns/blob/621ffde538e57772783f9e4a0df590d2f0d71beb/plugin/file/file.go#L133-L163

I think in addition to judging the "seenSOA", we should also judge whether zp.Next() exits normally or exits with an error.

	func Parse(f io.Reader, origin, fileName string, serial int64) (*Zone, error) {
	zp := dns.NewZoneParser(f, dns.Fqdn(origin), fileName)
	zp.SetIncludeAllowed(true)
	z := NewZone(origin, fileName)
	seenSOA := false
	for rr, ok := zp.Next(); ok; rr, ok = zp.Next() {
	if err := zp.Err(); err != nil {
	return nil, err
	}

	if !seenSOA {
	if s, ok := rr.(*dns.SOA); ok {
	seenSOA = true

	// -1 is valid serial is we failed to load the file on startup.

	if serial >= 0 && s.Serial == uint32(serial) { // same serial
	return nil, &serialErr{err: "no change in SOA serial", origin: origin, zone: fileName, serial: serial}
	}
	}
	}

	if err := z.Insert(rr); err != nil {
	return nil, err
	}
	}
	if !seenSOA {
	return nil, fmt.Errorf("file %q has no SOA record for origin %s", fileName, origin)
	}

	return z, nil

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plugin/file should not reload with wrong file #6697

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plugin/file should not reload with wrong file #6697

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