If filesystem in virtual router is in read-only state, reports error to
commands send to virtual router, instead of keeping silence.

Test:

Before change:
(1) Acquire IP. always in "Allocating" state.
(2) EnableStaticNat, the result is success(it is incorrect).
(3) DisableStaticNat, will get error message.. This is correct.
(4) Add Firewalls. always in "Adding" state.
(5) The AgentManager report statistics every 60 minutes(normally it
should be router.stats.interval=5 minutes).

After change:
(1) Acquire IP, will get error message.
(2) EnableStaticNat, will get error message.
(3) DisableStaticNat, will get error message.
(4) Add Firewalls, will get error message. But the firewall rules are
saved in database.
(5) The AgentManager report statistics every 5 minutes, except the
network with read-only FS virtual router.