Replace all log.Err calls with log.AuditErr

[benileo]

This PR replaces all calls to log.Err() with log.AuditErr() which has the same functionality.

Fixes #1543

[jsha]

I had forgotten that AuditErr takes an error rather than a string like the rest of these functions. That's a little awkward, and it encourages an antipattern of doing log.AuditErr(err) without adding any context. As you can see in this diff, most of the places we want to call it already use Sprintf to add some nice context.

As part of this change, could you change AuditErr to take a string like the other logging functions? And update the other call sites?

[benileo]

AuditErr now takes a string and all call sites have been updated.

[jsha]

LGTM

[cpu]

LGTM 🍄.