blank page/503 output viewing a deleted topic if you don't have rights

We had a patch for better error handling (the part with this code)
and a hygiene patch that got rid of $this->user. They didn't work
together well.

Bug: 73004
Change-Id: I9694b8a2f6c1c2c0680c546167c5eaaa145f092f
Merge "blank page/503 output viewing a deleted topic if you don't have rights"