Blobconverter - skip if model can't be downloaded

[Erol444]

Currently, if blobconverter can't download the model, everything will crash. So if blobconverter server is down, or if there's a corpo firewall that prevents this, users won't be able to use depthai-viewer.
Ideally, depthai-viewer would try to download the model, but if there's an error it should still start the viewer, just without any AI model selected.