it seems that your theme doesn't define any content_width variable, as per this page:
http://codex.wordpress.org/Content_Width
If you do not want to edit your theme, you can define content_width with a plugin like this one:
http://wordpress.org/plugins/custom-content-width/
However, I agree that Jetpack should not display the error when the theme misses the variable. We fixed this issue in r758489-plugins, and the fix will be included in the next Jetpack release.