Don't link to packages when they no longer exist

[mschnitzer]

Issue Description

When a package gets deleted, but the backend build status information is not updated yet, the Monitor page still lists the deleted package and links to it even if it doesn't exist anymore. Maybe you should check if the package actually exists before linking to it.

Expected Result

There's no link when the package doesn't exist anymore.

How to Reproduce

You have to hope that the backend is slow in order to reproduce it:

1. Create some packages in your project
2. Create a patchinfo
3. Delete the patchinfo again
4. Click the blocked state in the build results page of the project (to go to the monitor page)
5. Hope that the build status from the backend didn't realize yet that the package no longer exists so that it still shows it in state blocked

Further Information

The package patchinfo was already deleted, but was still shown in the blocked state and had a link to the package view (clicking the link results in a 404):