Fixing error message: URI {uri} does not exist

[Lucas-C]

Checklist

• All new and existing tests are passing
• [NA] (If adding features:) I have added tests to cover my changes
• [NA] (If docs changes needed:) I have updated the documentation accordingly.
• [NA] I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)

What changes does this Pull Request introduce?

Previous to this one-line fix, the following error message could be returned:

URI does not exist %s

Now the error message containes the actual URI:

URI {uri} does not exist

Why is this necessary?

To help debug problems! 🙂

[jelmer]

The change overall LGTM, thanks!

[jelmer]

What's going on with the failing e2e test?

[Lucas-C]

Seems like the CI pipeline fails due to "screenshots not matching".

But this PR should not have any visual impact.

Maybe the reference screenshots should be updated in a separate PR?
The process is detailed in this documentation section: https://github.com/isso-comments/isso/blob/master/docs/docs/technical-docs/testing-client.rst#screenshot-comparison