Improve message "Network is unavailable. Please contact administrator"

This trivial change provide more details for root administrators during troubleshooting.

See discussion #11980

Author: daviftorres

engine/orchestration/src/main/java/com/cloud/vm/VirtualMachineManagerImpl.java

@@ -935,7 +935,7 @@ public void start(final String vmUuid, final Map<VirtualMachineProfile.Param, Ob
             throw new CloudRuntimeException(String.format("Unable to start a VM [%s] due to [%s].", vmUuid, e.getMessage()), e).add(VirtualMachine.class, vmUuid);
         } catch (final ResourceUnavailableException e) {
             if (e.getScope() != null && e.getScope().equals(VirtualRouter.class)){
-                throw new CloudRuntimeException("Network is unavailable. Please contact administrator", e).add(VirtualMachine.class, vmUuid);
+                throw new CloudRuntimeException("The Guest Network is unavailable. Please contact administrator", e.getMessage()).add(VirtualMachine.class, vmUuid);
             }
             throw new CloudRuntimeException(String.format("Unable to start a VM [%s] due to [%s].", vmUuid, e.getMessage()), e).add(VirtualMachine.class, vmUuid);
         }