How to provide chai expect with custom error message for mocha unit test?

Every expect method accepts an optional parameter message:

expect(1).to.be.above(2, 'nooo why fail??');
expect(1, 'nooo why fail??').to.be.above(2);

So, in your case it should be:

expect(result.length)
  .to.be.greaterThan(0, "It should parse at least one sail out of the flatfile, but result is empty");