Fix zone parser error handling

[rideliner]

1. Why is this pull request needed and what does it do?

Error handling for the zone parser does not follow the guidance of the dns library.

After Next returns (nil, false), the Err method will return any error that occurred during parsing.

zp.Err() will only return nil while inside the loop.

2. Which issues (if any) are related?

None

3. Which documentation changes (if any) need to be made?

None

4. Does this introduce a backward incompatible change or deprecation?

May cause an error where previously the parser would have silently stopped parsing the file.