If thier project is an inspiration and you did not credit them then in my opinion that is a bit of an ethical problem. The other thing that comes to mind for me is that if you were inspired by their project why to contribute to it rather than create a copycat?
> Is copying parts of README enough to call for attribution to the original project?
> Is copying error message enough to call for attribution of the original project?
> Where is the line?
If you and them just had the same idea, and both created the same project, without knowing of the other one's wor, then it'd be no-harm-no-foul. If you saw what they were doing, and thought you could do it better, and did your own thing, that'd also be fine; being inspired by someone else doesn't make you a copy.
But copying actual chunks of their work, like the phrasing in the README or in error messages, would count as outright copying them, and thus, credit would be nice.
There's no concrete line in the sand, but I think it's clear you took more than just their general idea, here.