Checking if the image creation failed due to IsAlreadyExists didn't use
the error from ImageService.Create.
Error from ImageService.Create was stored in a separate variable and
later IsAlreadyExists checked the standard `err` variable instead of the
`createErr`.
As there's no need to store the error in a separate variable - just
assign it to err variable and fix the check.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>