Failing test in ci

[joaothallis]

Without modification of develop branch, one test is failing in ci.

Build: https://travis-ci.org/github/evadne/gen_magic/jobs/684948387

[hrefhref]

The current code uses magic_error which apparently can return very random text or just no text. My PR #13 fixes this by using magic_errno.

[evadne]

I think this is resolved on the develop branch but if not please reopen. Thanks!