t412 - depcheck crash: KeyError
I'v added try-except block around the critical area.
Kamil and I found out, that this error happens, when packagers push new
builds into Bodhi update, in the time of depcheck-run on the respective
update (to be more specific, after we query Koji for -pending builds,
and before we sum-up the build results to update results).
If this happens, we now add the build do problematic builds list (with
appropriate error message), which subsequently FAILs the update.