D3D: Store error message by std::string instead of const char*. These error strings are stored for recording later we can't depend on them still being alive so make a copy. BUG=962439 Change-Id: Ie9156c99b9ce79df4a11087bdcab92d980e959a4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1610078 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>