Check status code on onvif snapshot

[balloob]

Breaking change

Proposed change

Check the status code to make sure the snapshot url works.

Fixes #33796

I don't have an ONVIF camera so unable to test this. @frenck would you be able to test this?

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[MartinHjelmare]

The fetch function has inconsistent return values. We should return None explicitly in the two other exits of this function.