core: skip unit deserialization and move to the next one when unit_deserialize()...

core: skip unit deserialization and move to the next one when unit_deserialize() fails

If unit_deserialize() fails (because one read line is overly long), it returns
an error and we would have assumed that the next read would point to the next
unit to deserialize.

But instead unit_deserialize() can leave the file offset in the middle of a
line.

Therefore we need to ignore and skip the current unit in this case too.

While at it, move unit deserialization in a dedicated functions. That should
make the code easier to read.