Fail gracefully when libSavitar is not found

By catching the ImportError, this prevents the logs being clogged with exceptions in start and when the plugins preference page is touched.

plugins/3MFReader/__init__.py

@@ -1,9 +1,16 @@
 # Copyright (c) 2015 Ultimaker B.V.
 # Cura is released under the terms of the AGPLv3 or higher.
 from typing import Dict
+import sys
+
+from UM.Logger import Logger
+try:
+    from . import ThreeMFReader
+except ImportError:
+    Logger.log("w", "Could not import ThreeMFReader; libSavitar may be missing")
 
-from . import ThreeMFReader
 from . import ThreeMFWorkspaceReader
+
 from UM.i18n import i18nCatalog
 from UM.Platform import Platform
 catalog = i18nCatalog("cura")
@@ -14,30 +21,36 @@ def getMetaData() -> Dict:
         workspace_extension = "3mf"
     else:
         workspace_extension = "curaproject.3mf"
-    return {
+
+    metaData = {
         "plugin": {
             "name": catalog.i18nc("@label", "3MF Reader"),
             "author": "Ultimaker",
             "version": "1.0",
             "description": catalog.i18nc("@info:whatsthis", "Provides support for reading 3MF files."),
             "api": 3
-        },
-        "mesh_reader": [
+        }
+    }
+    if "ThreeMFReader" in sys.modules:
+        metaData["mesh_reader"] = [
             {
                 "extension": "3mf",
                 "description": catalog.i18nc("@item:inlistbox", "3MF File")
             }
-        ],
-        "workspace_reader":
-        [
+        ]
+        metaData["workspace_reader"] = [
             {
                 "extension": workspace_extension,
                 "description": catalog.i18nc("@item:inlistbox", "3MF File")
             }
         ]
-    }
+    
+    return metaData
 
 
 def register(app):
-    return {"mesh_reader": ThreeMFReader.ThreeMFReader(),
-            "workspace_reader": ThreeMFWorkspaceReader.ThreeMFWorkspaceReader()}
+    if "ThreeMFReader" in sys.modules:
+        return {"mesh_reader": ThreeMFReader.ThreeMFReader(),
+                "workspace_reader": ThreeMFWorkspaceReader.ThreeMFWorkspaceReader()}
+    else:
+        return {}

plugins/3MFWriter/__init__.py

@@ -1,30 +1,39 @@
 # Copyright (c) 2015 Ultimaker B.V.
 # Uranium is released under the terms of the AGPLv3 or higher.
+import sys
 
-from UM.i18n import i18nCatalog
+from UM.Logger import Logger
+try:
+    from . import ThreeMFWriter
+except ImportError:
+    Logger.log("w", "Could not import ThreeMFWriter; libSavitar may be missing")
 from . import ThreeMFWorkspaceWriter
-from . import ThreeMFWriter
+
+from UM.i18n import i18nCatalog
 
 i18n_catalog = i18nCatalog("uranium")
 
 def getMetaData():
-    return {
+    metaData = {
         "plugin": {
             "name": i18n_catalog.i18nc("@label", "3MF Writer"),
             "author": "Ultimaker",
             "version": "1.0",
             "description": i18n_catalog.i18nc("@info:whatsthis", "Provides support for writing 3MF files."),
             "api": 3
-        },
-        "mesh_writer": {
+        }
+    }
+
+    if "ThreeMFWriter" in sys.modules:
+        metaData["mesh_writer"] = {
             "output": [{
                 "extension": "3mf",
                 "description": i18n_catalog.i18nc("@item:inlistbox", "3MF file"),
                 "mime_type": "application/vnd.ms-package.3dmanufacturing-3dmodel+xml",
                 "mode": ThreeMFWriter.ThreeMFWriter.OutputMode.BinaryMode
             }]
-        },
-        "workspace_writer": {
+        }
+        metaData["workspace_writer"] = {
             "output": [{
                 "extension": "curaproject.3mf",
                 "description": i18n_catalog.i18nc("@item:inlistbox", "Cura Project 3MF file"),
@@ -32,7 +41,12 @@ def getMetaData():
                 "mode": ThreeMFWorkspaceWriter.ThreeMFWorkspaceWriter.OutputMode.BinaryMode
             }]
         }
-    }
+
+    return metaData
 
 def register(app):
-    return {"mesh_writer": ThreeMFWriter.ThreeMFWriter(), "workspace_writer": ThreeMFWorkspaceWriter.ThreeMFWorkspaceWriter()}
+    if "ThreeMFWriter" in sys.modules:
+        return {"mesh_writer": ThreeMFWriter.ThreeMFWriter(), 
+                "workspace_writer": ThreeMFWorkspaceWriter.ThreeMFWorkspaceWriter()}
+    else:
+        return {}