I was getting errors mirroring and updating:

    failed to connect to github.com: Connection timed out; class=Os (2)

and

    remote 'origin' already exists; class=Config (7); code=Exists(-4)

It turns out that the `git_url` field, which I had been using previously
to mirror and clone repositories, was changed from an "https://" URL to
a "git://" URL. Since Reflectub is not authorised to clone any GitHub
"git://" URLs, the connection timed out.

The URL change seems to have happened around March 2022, or at least
between December 2021 and April 2022.

The second error was caused by a previously-created repository existing
in the filesystem, but not being in the database as it hadn't been
correctly mirrored. For now, I've decided not to fix that problem and am
only fixing the URL issue.

The GitHub API also includes a `clone_url` field, which contains the
HTTPS clone URL. This fixes the timeout problem.

Add full definitions for our new error variant ideas.

Use a distinct variant and message in each error case in order to trace
errors to the line of code where they occur.

These variants should make it easier to trace where in the code that a
particular error occurred, and include a context-descriptive error
message.

And add a change log.

We don't need to set the `agefile` config value because
"info/web/last-modified" is already CGit's default value for the
setting.

Reflectub is designed to work specifically with CGit, but could work
with other Git web frontends. Make this more explicit in the
description.