Wrong error message when failed to verify Pub/Sub topic exists

[zhongduo]

Describe the bug
When the system failed to verify Pub/Sub topic exists, the error message in broker status is shown as before instead of real cause:

  - lastTransitionTime: "2020-09-09T13:23:24Z"
    message: 'Failed to verify Pub/Sub topic exists: %!w(*status.Error=&{0xc0024ffe60})'
    reason: TopicVerificationFailed

Expected behavior
The message should print out the detailed reason of the failure.

To Reproduce
Without a valid service account, create the GCP broker. Then check status part in the yaml of the broker.

Knative-GCP release version

Additional context
Add any other context about the problem here such as proposed priority

[zhongduo]

Experiment shows that %w does not invoke the Error() string method that the error type provides for proper struct formatting. Changing to %v fixed the problem. Example output after fix:

- lastTransitionTime: "2020-09-09T14:13:41Z"
    message: |-
      Failed to verify Pub/Sub topic exists: rpc error: code = Unauthenticated desc = transport: oauth2: cannot fetch token: 400 Bad Request
      Response: {"error":"invalid_grant","error_description":"Invalid JWT Signature."}
    reason: TopicVerificationFaile