Add nice error message
Generally, when using Remote API to push images there needs a http Header
X-Registry-Auth.
For compatibility if there was no authConfig header, everything will be
okay if a proper JSON-http-body was applied.
But when both X-Registry-Auth Header and the Body are missing, due to
the function of decode JSON, it will return an EOF error which was not
very clear to user.
So I think we can make the respone error be more nice.
Signed-off-by: Hu Keping <hukeping@huawei.com>