I had to shuffle a bit the order of initialization since we were
creating a folder for the crash logs, as well as one for the backups a
bit too early. So now I move errors_init() after configuration folder
creation/migration, and I create both these folders in this function
(especially since gimp_init_signal_handlers() is run even earlier).

For this later reason, I also check for backtrace_file and backup_path
being allocated in gimp_eek() since it is also possible for signals
happening before errors_init(). In such a case, we will simply bypass
the GUI error handler (terminal error handler still possible) and the
backup (anyway there is nothing to backup at this point).

I could also try to create these 2 directories at the last second, when
needed. But since we are trying to do the strict minimum during crash
handling, it is better to do whatever can be done earlier.