If the problem occurs at one of the first layer, we may end up loading
nearly nothing even though there may be a whole lot of other data in
good state. So instead keep track of the number of failed layer and
channel loading to still display an error popup at the end, because we
still need to alert the user something went wrong. Yet damages are
limited.

Also make sure that layer paths are fixed when some layers could not be
loaded, hence stored paths end up wrong.

(cherry picked from commit 2045fdd2)