Improve logging in order to help diagnosing issues

[marccarre]

Problem:
The following log statement does not say enough to be able to debug the actual issue.

ts=2018-01-10T21:16:55.42468337Z caller=warming.go:154 component=warmer err="requesting tags: errors:\ndenied: requested access to the resource is denied\nunauthorized: authentication required\n"

Solution:
Log details like:

• what image was Flux requesting tags for,
• from which repository,
• with which credentials,
• etc.

Where the issue occurred:

• Slack: https://weave-community.slack.com/archives/C4U5ATZ9S/p1515685389000781
• Full logs: https://weave-community.slack.com/files/U8S9JQP2S/F8QQX4MAL/logs-from-agent-in-weave-flux-agent-c65ccd47b-v99lh.txt

[rade]

I believe the error does bubble up and gets logged with a little bit more context:

ts=2018-01-10T21:10:06.053691237Z caller=images.go:89 component=daemon err="fetching image metadata for index.docker.io/contractlandstaff/helium-authentication-web: errors:\ndenied: requested access to the resource is denied\nunauthorized: authentication required\n"

Still doesn't tell you anything about the credentials used though.