When docker pull a non-existent repo, daemon will report "image xxx not found"
with an error code 500, which should be 404.
This commit add 404 "not found" mapping and refactor httpError function.
Signed-off-by: Zhang Wei <zhangwei555@huawei.com>
Upstream-commit: ab42a3a23a1276ccad874a13cf8604cb59124e10
Component: engine