Image conflict should return HTTP 409

[tacerus]

Is this a BUG REPORT or FEATURE REQUEST? (leave only one on its own line)

/kind feature

Description

Steps to reproduce the issue:

1. Attempt to delete image which is still being used by a container using DELETE /images/sha256:xxxx

Describe the results you received:

API returns HTTP 500.

Describe the results you expected:

Given the Docker API returns the more appropriate HTTP 409 (CONFLICT), I expected Podman to return 409 as well.

Additional information you deem important (e.g. issue happens only occasionally):

I think it would not only be more appropriate to use a HTTP response with the matching description, but I too think it would help compatibility with Docker API clients.
In my case I am using the Docker Java API, which would return a appropriate conflict exception on Docker's 409 response, whereas Podman's 500 results in a very generic server error.

[vrothberg]

@jwhonce PTAL

[tacerus]

Thank you so much, @rhatdan! Much appreciated.