Make script execute properly in the context of 'set -e'

A pending change in SVN will put "set -e" into net/instaweb/automatic/system_test.sh.  This means that the nginx-specific system test, which sources that, must protect itself from shell commands that might error out, by wrapping them in "check_not" or "check_error_code" or simply appending "|| true" to the pipe.