Editing repo via api 404

[Daedaluz]

Editing repo via

Fixes #1453

Description of the Change

This fixes the repo edit api by correcting the logic that checks if the target repo name already exists.
Also, replies with an error describing the issue instead of an empty 404 reply

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[cfiehe]

@neolynx
The change looks good to me. It solves the problem that updating an existing repo fails when you have the same repo name in the body of your put request. The logic before considered that there is always a change in the repo's name when the name field is part of the message body.