system: don't fail when Boot service isn't there

If the boot service isn't there, we shouldn't fail the response but omit
the Boot property.

Tested:
1. Service Validator passed on system without boot service
2. Service Validator passed on system with boot service

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Id3488f827736aa65d73e4a42082b279ca386ac3e
diff --git a/redfish-core/lib/systems.hpp b/redfish-core/lib/systems.hpp
index 7180b09..3315a26 100644
--- a/redfish-core/lib/systems.hpp
+++ b/redfish-core/lib/systems.hpp
@@ -930,6 +930,10 @@
         if (ec)
         {
             BMCWEB_LOG_DEBUG << "DBUS response error " << ec;
+            if (ec.value() == boost::asio::error::host_unreachable)
+            {
+                return;
+            }
             messages::internalError(aResp->res);
             return;
         }
@@ -1014,6 +1018,10 @@
         if (ec)
         {
             BMCWEB_LOG_DEBUG << "DBUS response error " << ec;
+            if (ec.value() == boost::asio::error::host_unreachable)
+            {
+                return;
+            }
             messages::internalError(aResp->res);
             return;
         }