Fixing bug when there is no icon on head

When there is no icon on the <head> the script would try to include a `null` image.

Requesting `mywebsite.com/section/hello/world` would result in the script trying to include `mywebsite.com/section/hello/null` as a image.

I understand that this library is about managing the icon, but it should not fail while doing it.

Also changed the errors to include the original message, that should help debugging.
Merge pull request #15 from renan/patch-1

Fixing bug when there is no icon on head