Refreshing collection when nothing in collection causes error

[DannyBrown-xx]

If you don't have anything in your collection, selecting File > Refresh collection causes a long wait followed by this error box.

In addition, this error could be better formatted for reading purposes.

[Godley]

Having looked into this again I've reasoned why this happens. What it's doing is pulling down a lot of XML from the interwebs, from sources like MuseScore, so that you can search in the search box and it'll pull results out of the model for online sources, then deleting the XML. Apparently, sometimes that XML isn't valid. I turned off validation originally because it was so ridiculously slow, but once all that's moved to another thread as in issue #320 I should probably put back validation for online sources.

I'm in debate whether the debug output should be hidden from the user or not because it's only really useful info for me, so maybe I should upload it to a remote debug service?