Ensure resource exists before update

[danielrbradley]

When performing a resource update, most resources use a PUT method. This PUT is the same as is used for most create methods. Therefore, if a resource has moved or been deleted and a refresh has not detected the drift, when the provider performs the PUT during an update, it will actually create a brand new resource while reporting to the user that it's only performed an update.

To avoid this ambiguity we should perform a HEAD or GET request before the PUT to ensure that the resource does indeed already exist as we expect. If the resource no longer exists then we should prompt the user to perform a refresh to rectify the drift.