Improve error message when someone tries to open a merged PR with deleted branch

[filiptronicek]

Bug description

When I tried opening a pull request which had its branch already merged and deleted, I got an error that really left me wondering what really happened.

Steps to reproduce

1. Go to a merged PR with a deleted branch
2. Open the URL with GitPod
3. See the error (attached in my comment)
   Error: Cannot read property 'repository' of null

Expected behavior

Perhaps something like "couldn't find that branch in this repo, it's been probably deleted!

Example repository

More of an example Pull Request, here is one: #3431

[jankeromnes]

I agree that Gitpod should show a better error message here, so I'll try to fix it.

Also, fun fact, this bug was originally reported as #4

[jankeromnes]

Fixed in #3869, will go live with the next deployment.