This gives us better control over the error returned by the init
routine. Also changes the streaming buffer in VertexDataManager to not
be a unique_ptr.

Bug: angleproject:2738
Change-Id: I3193840dfb71c7574adbe65bc5f9227f4add1fd3
Reviewed-on: https://chromium-review.googlesource.com/1151448Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>