internal/worker: make log messages clearer
internal/worker: make log messages clearer

Make log messages clearer by logging the function that finished when
logging successes.

Change-Id: I5f60683628c6a37c878d3c60208fa7f2d29156c9
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/719982
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite