Reviewer is obliged to find all errors.
Not true. A reviewer’s main job is to give a high-level assessment on the quality of a paper. If the assessment is negative then usually they do not look for all specific errors in the paper. A detailed list of errors is more common when the reviewer recommends the journal to accept the paper (since then the author(s) can edit the paper and then publish in the journal) but still many reviewers do not do this (which is why it is common to find peer-reviewed papers with errors in them).
At least, this is the case in math.
Yes, but even in the case of a negative review they often demonstrate the cause by pointing on the several errors, or by listing some high-level reason why they are negative and it could be used as some form of the feedback.