Handles non-existing objects in _copy_saved_objects API call (#158036)

Closes #156791

## Summary

This PR implements catching the error thrown by the
saved_objects_exporter when an object is not found, and responding with
a detailed 404 ("Not Found") rather than a generic 500 ("Internal Server
Error") response message.

### Example Response:
```
{
  "statusCode": 404,
  "error": "Not Found",
  "message": "Saved objects not found",
  "attributes": {
    "objects": [
      {
        "id": "7adfa750-4c81-11e8-b3d7-01146121b73d",
        "type": "dashboard"
      },
      {
        "id": "571aaf70-4c88-11e8-b3d7-01146121b73d",
        "type": "search"
      },
      {
        "id": "d3d7af60-4c81-11e8-b3d7-01146121b73d",
        "type": "index-pattern"
      }
    ]
  }
}
```

### Testing

1. Install sample flight data, find the id of the flights dashboard SO
2. Create an additional space 'b'
3. Issue a request to copy saved objects to space b
```
POST kbn:/api/spaces/_copy_saved_objects
{
  "spaces": [
    "b"
  ],
  "objects": [
    {
      "type": "dashboard",
      "id": "7adfa750-4c81-11e8-b3d7-01146121b73e"
    },
    {
      "type": "dashboard",
      "id": "7adfa750-4c81-11e8-b3d7-01146121b73f"
    },
    {
      "type": "dashboard",
      "id": "7adfa750-4c81-11e8-b3d7-01146121b73g"
    }
  ]
}
```
4. Verify response
```
{
  "statusCode": 404,
  "error": "Not Found",
  "message": "Saved objects not found",
  "attributes": {
    "objects": [
      {
        "type": "dashboard",
        "id": "7adfa750-4c81-11e8-b3d7-01146121b73e"
      },
      {
        "type": "dashboard",
        "id": "7adfa750-4c81-11e8-b3d7-01146121b73f"
      },
      {
        "type": "dashboard",
        "id": "7adfa750-4c81-11e8-b3d7-01146121b73g"
      }
    ]
  }
}
```
5. Issue a request to copy the flights dashboard SO
6. Verify the usual response (200, missing references)
7. Issue a malformed request to copy an SO
8. Verify status 400 response with details

docs/api/spaces-management/update_objects_spaces.asciidoc

@@ -36,6 +36,15 @@ Updates one or more saved objects to add and/or remove them from specified space
 `spacesToRemove`::
   (Required, string array) The IDs of the spaces the specified objects should be removed from.
 
+[[spaces-api-update-objects-spaces-response-codes]]
+==== Response codes
+
+`200`::
+    Indicates a successful call.
+
+`404`::
+    Indicates that the request failed because one or more of the objects specified could not be found. A list of the unresolved objects are included in the 404 response attributes.
+
 [role="child_attributes"]
 [[spaces-api-update-objects-spaces-response-body]]
 ==== {api-response-body-title}

x-pack/plugins/spaces/server/routes/api/external/copy_to_space.ts

@@ -105,19 +105,35 @@ export function initCopyToSpacesApi(deps: ExternalRouteDeps) {
         })
       );
 
-      const copySavedObjectsToSpaces = copySavedObjectsToSpacesFactory(
-        startServices.savedObjects,
-        request
-      );
-      const sourceSpaceId = getSpacesService().getSpaceId(request);
-      const copyResponse = await copySavedObjectsToSpaces(sourceSpaceId, destinationSpaceIds, {
-        objects,
-        includeReferences,
-        overwrite,
-        createNewCopies,
-        compatibilityMode,
-      });
-      return response.ok({ body: copyResponse });
+      try {
+        const copySavedObjectsToSpaces = copySavedObjectsToSpacesFactory(
+          startServices.savedObjects,
+          request
+        );
+        const sourceSpaceId = getSpacesService().getSpaceId(request);
+        const copyResponse = await copySavedObjectsToSpaces(sourceSpaceId, destinationSpaceIds, {
+          objects,
+          includeReferences,
+          overwrite,
+          createNewCopies,
+          compatibilityMode,
+        });
+        return response.ok({ body: copyResponse });
+      } catch (e) {
+        if (e.type === 'object-fetch-error' && e.attributes?.objects) {
+          return response.notFound({
+            body: {
+              message: 'Saved objects not found',
+              attributes: {
+                objects: e.attributes?.objects.map((obj: SavedObjectIdentifier) => ({
+                  id: obj.id,
+                  type: obj.type,
+                })),
+              },
+            },
+          });
+        } else throw e;
+      }
     })
   );