This pattern isn't available when using angle::Result instead of
gl::Error. Refactor the Buffer11 class to use gl::Error instead. This
unblocks further return value refactoring for perf.

Bug: angleproject:2738
Change-Id: I739044ae318c6b1fc40756a4f1fcadf609276c1a
Reviewed-on: https://chromium-review.googlesource.com/1142952Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>