lazy initialization for PlasmaDownloader to reduce ZKL startup time by 1 second

EDIT: want to delay this PR a bit. It didn't reduce startup time because ZKL have to wait for rapid list to download (so that first time user auto download ZK). 

At least ZKL show UI a bit sooner before being stuck initializing PlasmaDownloader.

Also, non-critical:
I reduce the verbose-ness of "repositories.dat" file-not-found and "selected.list" file-not-found error message.