frontend, backend: fix traceback for non-existing tasks

[FrostyX]

Fix #2390

First, take a look at the frontend change. The BuildsLogic.get_srpm_build_task never raises an exception. It returns None for non-existing tasks instead. The route therefore returns 200 status code even for non-existing tasks, and simply returns nil. I am changing this to return 404 instead.

This prompted the backend changes because exceptions started appearing.

[praiskup]

Would you mind bumping MIN_FE_BE_API?

[praiskup]

Why backend even asked for those build IDs?

[FrostyX]

Why backend even asked for those build IDs?

I am thinking of something like

1. User submitted a build
2. Backend added it to its priority queue
3. Because of one of the post-release bugs, the building was stuck
4. User deleted the build because it wasn't working
5. Before updating the priority queue, backend picked the build and started it

I need to check if this was the case and if this situation can happen. If yes, we should fix it.

[FrostyX]

I am thinking of something like

Now it looks more like an issue caused by #2398.

[praiskup]

So are we going to skip this PR then?

[FrostyX]

We should probably skip this for the next mini-release. But the change is valid, I think.

[praiskup]

Thank you for this patch, it makes the code more robust..