Enable errorlint and fix existing issues

[markusthoemmes]

As per title.

/assign @nak3 @tcnghia

[codecov]

Codecov Report

Merging #262 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
- Coverage   95.24%   95.12%   -0.13%     
==========================================
  Files          36       37       +1     
  Lines         694      697       +3     
==========================================
+ Hits          661      663       +2     
- Misses         23       24       +1     
  Partials       10       10              

Impacted Files	Coverage Δ
pkg/apis/networking/v1alpha1/register.go	100.00% <0.00%> (ø)
.../apis/networking/v1alpha1/domainclaim_lifecycle.go	0.00% <0.00%> (ø)

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[tcnghia]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markusthoemmes, tcnghia

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