CLC-3150 Fix timeout log messages for Canvas proxies

https://jira.media.berkeley.edu/jira/browse/CLC-3150

We expect to make other changes to this stretch of code soon, but first we want to make the existing code work as designed.

I've also added WARN-level logging of "success-with-messages" statuses from the Canvas import, since they tell us about rows that were missed (e.g., student enrollments which were not added).