Uniquify unsupported distro error message

Change a distro-not-supported error message in lib/horizon so that it
can't be confused with a similar error case in lib/apache.

Change-Id: I1197cb4de1497906e93a2c3ce09c3c06afe03b65
diff --git a/lib/horizon b/lib/horizon
index 5973eb2..f770ded 100644
--- a/lib/horizon
+++ b/lib/horizon
@@ -131,7 +131,7 @@
         fi
         sudo sed '/^Listen/s/^.*$/Listen 0.0.0.0:80/' -i /etc/httpd/conf/httpd.conf
     else
-        exit_distro_not_supported "apache configuration"
+        exit_distro_not_supported "horizon apache configuration"
     fi
 
     # Remove old log files that could mess with how devstack detects whether Horizon