[SDK] Raise more human-readable name conflict exception

[droctothorpe]

What this PR does / why we need it:
If you call KatibClient.tune using an experiment name that already exists, you get a giant traceback with the relevant / helpful message not being at the bottom. We had a user struggle to debug this on their own.

This PR simply raises a more helpful exception in the event of a 409 (name conflict) response from the API.

Checklist:

• Docs included if any changes are user facing

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[droctothorpe]

/assign @andreyvelich

[andreyvelich]

Thank you for updating this @droctothorpe
/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andreyvelich, droctothorpe

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[andreyvelich]

@droctothorpe Please can you rebase your PR, it should fix the E2Es.

[droctothorpe]

@droctothorpe Please can you rebase your PR, it should fix the E2Es.

Squashed and rebased @andreyvelich. Thank you!

[tenzen-y]

Thank you!
/lgtm