Suppress error output for missing variables

This condition is expected and handled by the code, so there is no
need to write error logs when it happens.

salt/modules/sysrc.py

@@ -49,9 +49,8 @@ def get(**kwargs):
     else:
         cmd += " -a"
 
-    sysrcs = __salt__["cmd.run"](cmd)
+    sysrcs = __salt__["cmd.run"](cmd=cmd, output_loglevel="quiet")
     if "sysrc: unknown variable" in sysrcs:
-        # raise CommandExecutionError(sysrcs)
         return None
 
     ret = {}