gs: add more logger scopes on a few handlers

Created by: jhchabran

While looking at https://sentry.io/organizations/sourcegraph/issues/3381789776/?project=5596888&query=is%3Aunresolved, I found that these errors have basically no scope outside the root one. Overall it seems that the scopes are a bit underutilized in Gitserver.

This PR is a small suggestion to possibly get started on this as this would help.

Two things to note:

• There is this PR https://github.com/sourcegraph/log/pull/13 to stop sending transient errors to Sentry
• We could use scopes to gauge error rates, including on the transient ones, enabling to have a better use of the the errors signal.

➡️ Thoughts?

Test plan

n/a logging changes and ci should cover it.