Improve transform error output

This change makes it so that the error message itself is printed on the
same line as the report status and file name. This makes it easier for
other tools to process the output.

src/Worker.js

@@ -163,7 +163,7 @@ function run(data) {
           updateStatus(
             'error',
             file,
-            'Transformation error\n' + trimStackTrace(err.stack)
+            'Transformation error ('+ err.message.replace(/\n/g, ' ') + ')\n' + trimStackTrace(err.stack)
           );
           callback();
         }