Catch error when qemuy can't connect to his console

Fix #803

gns3server/compute/qemu/qemu_vm.py

@@ -852,8 +852,6 @@ class QemuVM(BaseNode):
                 yield from self.resume()
                 return
 
-            else:
-
             if self._manager.config.get_section_config("Qemu").getboolean("monitor", True):
                 try:
                     info = socket.getaddrinfo(self._monitor_host, 0, socket.AF_UNSPEC, socket.SOCK_STREAM, 0, socket.AI_PASSIVE)
@@ -908,7 +906,10 @@ class QemuVM(BaseNode):
 
             if "-enable-kvm" in command_string:
                 self._hw_virtualization = True
+        try:
             yield from self.start_wrap_console()
+        except OSError as e:
+            raise QemuError("Could not start QEMU console {}\n".format(e))
 
     def _termination_callback(self, returncode):
         """