Slackreporter: Log channel on reporting errors
Slackreporter: Log channel on reporting errors

We have a bunch of errors because the channel wasn't found, but the
error doesn't include the info which channel this is about:

```
failed to write Slack message: request failed: channel_not_found
```

michelle192837/test-infra,cblecker/test-infra,kubernetes/test-infra,fejta/test-infra,cjwagner/test-infra,dims/test-infra,cjwagner/test-infra,BenTheElder/test-infra,monopole/test-infra,monopole/test-infra,michelle192837/test-infra,jessfraz/test-infra,cblecker/test-infra,BenTheElder/test-infra,fejta/test-infra,monopole/test-infra,cblecker/test-infra,dims/test-infra,cblecker/test-infra,kubernetes/test-infra,jessfraz/test-infra,fejta/test-infra,michelle192837/test-infra,jessfraz/test-infra,monopole/test-infra,dims/test-infra,dims/test-infra,fejta/test-infra,fejta/test-infra,jessfraz/test-infra,BenTheElder/test-infra,kubernetes/test-infra,fejta/test-infra,cjwagner/test-infra,michelle192837/test-infra,monopole/test-infra,kubernetes/test-infra,cjwagner/test-infra,michelle192837/test-infra,kubernetes/test-infra,BenTheElder/test-infra,monopole/test-infra,cblecker/test-infra,kubernetes/test-infra,BenTheElder/test-infra,cjwagner/test-infra,jessfraz/test-infra,dims/test-infra,BenTheElder/test-infra,cblecker/test-infra,jessfraz/test-infra,dims/test-infra,michelle192837/test-infra,cjwagner/test-infra