Print exception stack when generating a test case fails.

test/generate-testcase.js

@@ -127,6 +127,7 @@ function runReadability(source, destPath, metadataDestPath) {
     var result = new Readability(uri, doc).parse();
   } catch (ex) {
     console.error(ex);
+    ex.stack.forEach(console.log.bind(console));
   }
   if (!result) {
     console.error("No content generated by readability, not going to write expected.html!");