Update download project error messages

src/packager/download-project.js

@@ -289,9 +289,12 @@ const downloadProject = async (data, progressCallback) => {
         zip = await JSZip.loadAsync(data);
       } catch (e) {
         console.warn(e);
-        throw new Error('Cannot parse project');
+        throw new Error('Cannot parse project: not a zip or sb');
       }
       const projectDataFile = zip.file('project.json');
+      if (!projectDataFile) {
+        throw new Error('project.json is missing');
+      }
       const projectDataText = await projectDataFile.async('text');
       const projectData = JSON.parse(projectDataText);
       type = identifyProjectType(projectData);