Skip running behaviors when page is 4xx or 5xx
Skip running behaviors when page is 4xx or 5xx

Currently, when we run `Browser.browse_page`, we run JS behaviors after
we navigate to a page regardless of its status.
Maybe the page wasn't found (4xx) or unreachable for any reason (5xx).
In that case, we could skip running behaviors to save time and
resources.

With this PR, we add a new var to store navigated page HTTP status in
`WebsockReceiverThread.page_status`. We use this in
`Browser.browser_page` to skip behaviors, outlink and hashtag extraction
when page status is 4xx/5xx.

Note that we don't skip screenshots as it could be useful to have a
picture of an error page in some cases.

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