Landing page: Fix error_log nginx directive
Landing page: Fix error_log nginx directive

The 'off' option available in access_log is not available
for error_log, causing the output of error logging to be sent
to a file called 'off'. We can fix by setting output file to
/dev/null.

heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop