Forbid controller and compute servers to be different versions.

Report last compute server error to clients and display in the server summary.

tests/controller/test_compute.py

@@ -277,6 +277,7 @@ def test_json(compute):
         "cpu_usage_percent": None,
         "memory_usage_percent": None,
         "connected": True,
+        "last_error": None,
         "capabilities": {
             "version": None,
             "node_types": []