Handle HTTP errors when registering with Panopticon

Because detailed guides live at /, we occasionally get slug clashes when
registering with Panopticon. Prior to this change the script would just
die, now we print a warning and full error and continue.
Send HTTPErrorResponse to error level
Merge pull request #1255 from alphagov/handle-errors-in-panopticon-registration

Handle HTTP errors when registering with Panopticon