Fix syncing materials via API, show nice message

plugins/UM3NetworkPrinting/src/Models/Http/ClusterMaterial.py

@@ -0,0 +1,16 @@
+# Copyright (c) 2019 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+from ..BaseModel import BaseModel
+
+
+class ClusterMaterial(BaseModel):
+    def __init__(self, guid: str, version: int, **kwargs) -> None:
+        self.guid = guid  # type: str
+        self.version = version  # type: int
+        super().__init__(**kwargs)
+
+    def validate(self) -> None:
+        if not self.guid:
+            raise ValueError("guid is required on ClusterMaterial")
+        if not self.version:
+            raise ValueError("version is required on ClusterMaterial")