[7.17] Handles non-existing objects in _copy_saved_objects API call (#158036) (#158415)

# Backport

This will backport the following commits from `main` to `7.17`:
- [Handles non-existing objects in _copy_saved_objects API call
(#158036)](https://github.com/elastic/kibana/pull/158036)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

docs/api/spaces-management/copy_saved_objects.asciidoc

@@ -63,6 +63,15 @@ NOTE: This cannot be used with the `overwrite` option.
 +
 NOTE: This cannot be used with the `createNewCopies` option.
 
+[[spaces-api-copy-saved-objects-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-copy-saved-objects-response-body]]
 ==== {api-response-body-title}

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

@@ -94,18 +94,34 @@ export function initCopyToSpacesApi(deps: ExternalRouteDeps) {
         usageStatsClient.incrementCopySavedObjects({ headers, createNewCopies, overwrite })
       );
 
-      const copySavedObjectsToSpaces = copySavedObjectsToSpacesFactory(
-        startServices.savedObjects,
-        request
-      );
-      const sourceSpaceId = getSpacesService().getSpaceId(request);
-      const copyResponse = await copySavedObjectsToSpaces(sourceSpaceId, destinationSpaceIds, {
-        objects,
-        includeReferences,
-        overwrite,
-        createNewCopies,
-      });
-      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,
+        });
+        return response.ok({ body: copyResponse });
+      } catch (e: any) {
+        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;
+      }
     })
   );