Added useful error message for invalid line

[chrisspen]

If a zone file contains an invalid line, an zone_file.exceptions.InvalidLineException exception is thrown, but it doesn't say what's wrong with the line or which line it is, leaving the developer to dig through the code looking to an explanation.

[shea256]

Hey @chrisspen thanks for reporting. We'll look at this soon. Also curious what you use this library for.

[chrisspen]

Thanks. I'm using it, along with an API, to automate some DNS updates, so I don't have to manually login to my DNS provider's website and manually update records.