raise a proper docutils error when the request to get a gist url fails

Without this change, I was unable to complete a `nikola build` of the demo site when on my laptop disconnected from the internet.
be explicit

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
documentation

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Merge pull request #1402 from pwm1234/master

raise a proper docutils error when the request to get a gist url fails