Bonsai --export-image can blocks due to failures from restoring the environment

[PathogenDavid]

This happens when generating documentation of bonsai-rx/machinelearning@789b1b8

Bonsai shows the NuGet restore dialog with an error and blocks waiting for the user to interact with the UI to dismiss the error.

This isn't good or useful in a command line context. In a CI context, it causes the run to hang since there's no interactive user to even see the dialog.

The error is printed to the terminal already, which is all that's really needed in a context where Bonsai is being run from a script (which is what --export-image is for.)