CircleCI make sure reference screenshots exist
CircleCI make sure reference screenshots exist

The screenshot test doesn't error if the reference doesn't exist. Therefore
when introducing a new screenshot, but forgetting to upload the reference would
originally still make the build pass. This check prevents that.


Former-commit-id: 95be2b247ac4a3fe8d490813afef51a5cfa2b3d1
cBioPortal/cbioportal-frontend,cBioPortal/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,alisman/cbioportal-frontend