First of all: Thanks for the feedback, I know there's no progress indicator or anything like that, not needed for development phase
I plan to split the process to a few steps just to avoid running into php execution time limits or something like that.
If you get an error during the process it should either be in the syslog/web log (if it is a php error) or on the interface (db error). I have not yet come to the point where I needed further error info because the import steps are not ready, yet.
As you state that you got no ftp user import, mail accounts cannot be created because they come after ftp. Either there was en error in dns import or ftp user import I think.