cgmanager-proxy: hacky check for error from server in get_value
cgmanager-proxy: hacky check for error from server in get_value

this needs to be cleaned up and errors nicely propagated to the
client, and done so for every dbus call served by the proxy.

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

udeved/cgmanager,lxc/cgmanager,hustcat/cgmanager,hallyn/cgmanager-pkg-ubuntu,udeved/cgmanager,hallyn/cgmanager,hustcat/cgmanager,hallyn/cgmanager-pkg-ubuntu,hallyn/cgmanager,lxc/cgmanager