Create placeholder empty node for projects that failed to load

[MattReimer]

Update: This can be a low-priority item for QRAVE 0.7. Everything else should take precedence.

This is related to the fix for #40

A nice feature to have as well is to make sure to always render the root node no matter what so you have something to grab onto and close. This way there's not a possibility of having a hidden project you can't close.

For 0.6.0 we flat-out refuse to load the layer and clean it up if it won't parse (with good errors of course) but it could be better.