This PR was merged into the 2.0.x-dev branch.

Discussion
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Update "Error Handlers" part in usage.rst

Update "Error Handlers" part in usage.rst because of https://github.com/silexphp/Silex/commit/e96f9be8fda96193313bb655d63b061d6d4d7c4d#diff-9460b640f6466d5890f37f7caf262024R52

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46f125c4 Update "Error Handlers" part in usage.rst