Reduce nested #capture_git_error calls on Wiki

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What does this MR do and why?

This MR refactors the wiki model to make #capture_git_error private in order to solve the problem raised in the issue #468185:

As far as I could see in the code base, I still think that we can make the #capture_git_error private, as it being used outside of the WikiPage seems like a leak of internals IMHO. This should be quite straightforward because:

• Currently, WikiPages::CreateService calls page.create from within a capture_git_error, but the page.create already uses the capture_git_error internally, so I don't see a reason why we're using it twice.
• Same thing happens on WikiPages::UpdateService, it's calling page.update from within a capture_git_error, but the page.update already calls the capture_git_error internally, therefore I think we can remove the external call as well.

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