flux diff kustomization keeps running after reporting an error

Describe the bug

The diff worker keeps going after sending an error on errChan. It prints the output and tries to send again, but the caller only receives once. So we can end up writing output after the command returns, then blocking on a send nobody will receive.

Steps to reproduce

On dede56a8fdcd1e0edca28630a979a51415bb56f8, with the test prerequisites installed:

GOTOOLCHAIN=go1.27.1 make test \
  TEST_ARGS='-run ^TestDiffKustomizationNewNamespaceAndConfigmap -race -count=3'

This can report a race between executeCommand() reading the output buffer and the worker writing to it. It doesn't necessarily trigger on every run. The ConfigMap error itself is expected, but continuing after reporting it isn't.

Expected behavior

Print the partial diff before returning the original error, then stop the worker.

I've moved cmd.Print(output) before the error handling and added a return after the send in this PR.

Screenshots and recordings

No response

OS / Distro

Ubuntu 26.04.1 LTS on WSL2, Go 1.27.1.

Flux version

N/A - source checkout at dede56a8fdcd1e0edca28630a979a51415bb56f8.

Flux check

N/A - reproduced with envtest, without a Flux installation.

Git provider

No response

Container Registry provider

No response

Additional context

No response

Code of Conduct

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