Improve Job awaiter error and warning reporting

[metral]

Problem description

The structure of warning messages during a job await is not intuitive when failures or errors arise. Consider rearranging how the message is structured to inform the user of the diagnostics seen up to now, instead of the warning alone.

Errors & Logs

Affected product version(s)

#633 / soon-to-be master

Suggestions for a fix

#633 (comment)

[lblackstone]

Appending warnings to the status message would have required nontrivial changes, so I decided to limit the scope to the most common case: containers with unready status

Since this "warning" is a normal part of every Pod's lifecycle, I skipped printing it unless the Job failed to become ready. This cuts down on noise, and should only print warnings that adversely impact readiness.