[release-0.20] Fix race on error channel in consumer factory

[knative-prow-robot]

This is an automated cherry-pick of #364

🐛 Fix race on error channel in consumer factory

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-sandbox-eventing-kafka-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/common/consumer/consumer_factory.go	81.8%	84.8%	3.0

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-0.20@900179e). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             release-0.20     #368   +/-   ##
===============================================
  Coverage                ?   76.05%           
===============================================
  Files                   ?      114           
  Lines                   ?     4398           
  Branches                ?        0           
===============================================
  Hits                    ?     3345           
  Misses                  ?      854           
  Partials                ?      199           

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[slinkydeveloper]

/lgtm
/approve

[matzew]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, matzew, slinkydeveloper

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[matzew]

Ah, damn CLA bot

[slinkydeveloper]

@googlebot I consent.