LoadImage checked the immediate ImageLoad error, but then discarded the
response body.
The daemon can report load failures in that JSON message stream, so
tests could miss a structured load error and continue after a failed
image load.
Parse the ImageLoad response with jsonmessage.DisplayStream and discard
only the rendered output, so streamed errors fail the test.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>