fix(backend): Fixes response status of http error code when uploading duplicate pipeline [Fixes #10311]

[champon1020]

Description of your changes:
This is the backend fix of #10311.
I propose following solution but feel free to recommend a better one.

When uploading pipeline which already exists, it returns HTTP 500 error (internal server error) while it should return 409 error (conflict error). In order to solve this problem, I insert a branch by error code using util.IsUserErrorCodeMatch.

Checklist:

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[google-oss-prow]

Hi @champon1020. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[rimolive]

Thank you for your contribution, @champon1020!

/ok-to-test

[rimolive]

/lgtm

After uploading the same pipeline twice I got this expected error from API:

{"error_message":"Failed to create a pipeline and a pipeline version. The pipeline already exists.: Failed to create a pipeline and a pipeline version: Already exist error: Failed to create a new pipeline. The name some_name already exists. Please specify a new name","error_details":"Failed to create a pipeline and a pipeline version. The pipeline already exists.: Failed to create a pipeline and a pipeline version: Already exist error: Failed to create a new pipeline. The name some_name already exists. Please specify a new name"}

[rimolive]

@chensun @Tomcli I confirm this PR is working. Please review/approve.

[Tomcli]

/lgtm

[Tomcli]

/approve

[google-oss-prow]

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[rimolive]

/test kubeflow-pipeline-e2e-test