My layman's understanding is that the threshold for launching a counterintelligence investigation is rather low. The FBI gets a whiff of suspicious behavior, or a report from a source thought to be credible, and it checks it out. It may well be that Horowitz acknowledged this fact, and that this is what the leakers and the media are madly spinning.
BUT: then we come to "errors and omissions." An American citizen can't simply be assumed to be guilty and surveilled without evidence. Rumor, innuendo, and uncorroborated hearsay aren't enough. The question is not whether the opening of a probe was warranted; the question is whether the move to surveillance and wiretapping was warranted. That is a separate question.
If this is all about Carter Page -- Naval Academy grad, former Navy intelligence officer, a past target of Russian recruitment efforts, a cooperator with the FBI in nailing said Russians, etc. -- one should assume, absent some smoking gun, that he was a good guy. A counterintelligence probe should have begun by interviewing Page himself. That wasn't done. He was treated as a hostile target from the beginning, apparently solely on the basis of the Steele dossier.
So: Horowitz could well conclude that there was a basis for opening an investigation, but that there was no basis for surveillance, for wiretapping, for four warrants from the FISA court, etc. A decision to go Inspector Javert on an American citizen based on an unsubstantiated Steele dossier shouldn't be minimized as a mere error. And misrepresentation of the Steele dossier to the FISA Court, the claim that it was verified, and failure to disclose that it was a democrat oppo research trash heap shouldn't be dismissed as a mere "omission."
Horowitz's words will need to be parsed very carefully. He may well be an agency lifer who is attempting to minimize the scandal. He may well provide language that the democrats and the media can exploit. But the facts will stand independently of tricks of wording.