Fail the upload action if uploading a folder with no SARIF files in.

lib/upload-lib.js

@@ -61,6 +61,9 @@ async function upload(input) {
         const sarifFiles = fs.readdirSync(input)
             .filter(f => f.endsWith(".sarif"))
             .map(f => path.resolve(input, f));
+        if (sarifFiles.length === 0) {
+            core.setFailed("No SARIF files found to upload in \"" + input + "\".");
+        }
         await uploadFiles(sarifFiles);
     }
     else {