Fix golangci-lint issues

[redbeam]

Fixes golangci-lint issues; they're mostly about checking Fprintf for errors and handling errors in deferred functions.

[DeedleFake]

Thank you for the pull request.

I do not want all of those extra _, _ = before every single Fprint*() call. I do not agree with golangci-lint on this in the slightest.

The defer changes and the embedded redundancy removal are probably fine. Remove the _, _ and I'll take another look.

[redbeam]

Yeah, I get your point. The other option would be to silence them using //nolint:errcheck, but I'm not sure if that's any better.

Anyway, I dropped the commit.