branchmap: display filtername when `updatebranch` fails to do its jobs

We have a very handy assert at the ends of `branchmap.updatecache` that check
the resulting branchmap is actually valid.

I know we do not like assert in mercurial but this one is very handy for
debugging. There is really not reason for `branchmap.updatecache` to have this
kind of issue but this happened and handful of time during the development of
this or introduction of other related feature. I advice to keep it around until
we are a bit more confident with the new code.