Fix error handling of ID3D11DeviceContext::Map Refactors error handling should map() fail to avoid crashing during a device error event. BUG=angleproject:2284 Change-Id: Ib1eb676860fa5c3a81e523f49aab82b7e6700f73 Reviewed-on: https://chromium-review.googlesource.com/826485 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>