filter out "missing" thumbnails in tapor import
some tapor-imported tools have thumbnail: "http://tapor.ca/images/tools/missing.jpg" -- can we skip those?
e.g. https://sshoc-marketplace.acdh-dev.oeaw.ac.at/tools/239
some tapor-imported tools have thumbnail: "http://tapor.ca/images/tools/missing.jpg" -- can we skip those?
e.g. https://sshoc-marketplace.acdh-dev.oeaw.ac.at/tools/239