Fix server crash when deleting folders

Handle the case where a folder's parent path no longer exists when
processing VFS removal events. This can happen when a folder and its
contents are deleted together, causing a race between removal events.

Added a regression test for this scenario.

rojo-test/serve-test-snapshots/end_to_end__tests__serve__remove_folder_all-2.snap

@@ -0,0 +1,16 @@
+---
+source: tests/tests/serve.rs
+expression: "read_response.intern_and_redact(&mut redactions, root_id)"
+---
+instances:
+  id-2:
+    Children: []
+    ClassName: Folder
+    Id: id-2
+    Metadata:
+      ignoreUnknownInstances: false
+    Name: remove_folder
+    Parent: "00000000000000000000000000000000"
+    Properties: {}
+messageCursor: 1
+sessionId: id-1

rojo-test/serve-test-snapshots/end_to_end__tests__serve__remove_folder_all.snap

@@ -0,0 +1,38 @@
+---
+source: tests/tests/serve.rs
+expression: "read_response.intern_and_redact(&mut redactions, root_id)"
+---
+instances:
+  id-2:
+    Children:
+      - id-3
+    ClassName: Folder
+    Id: id-2
+    Metadata:
+      ignoreUnknownInstances: false
+    Name: remove_folder
+    Parent: "00000000000000000000000000000000"
+    Properties: {}
+  id-3:
+    Children:
+      - id-4
+    ClassName: Folder
+    Id: id-3
+    Metadata:
+      ignoreUnknownInstances: false
+    Name: my-folder
+    Parent: id-2
+    Properties: {}
+  id-4:
+    Children: []
+    ClassName: StringValue
+    Id: id-4
+    Metadata:
+      ignoreUnknownInstances: false
+    Name: test
+    Parent: id-3
+    Properties:
+      Value:
+        String: "This file is inside my-folder\r\n"
+messageCursor: 0
+sessionId: id-1

rojo-test/serve-test-snapshots/end_to_end__tests__serve__remove_folder_info.snap

@@ -0,0 +1,13 @@
+---
+source: tests/tests/serve.rs
+expression: redactions.redacted_yaml(info)
+---
+expectedPlaceIds: ~
+gameId: ~
+placeId: ~
+projectName: remove_folder
+protocolVersion: 5
+rootInstanceId: id-2
+serverVersion: "[server-version]"
+sessionId: id-1
+unexpectedPlaceIds: ~

rojo-test/serve-test-snapshots/end_to_end__tests__serve__remove_folder_subscribe.snap

@@ -0,0 +1,13 @@
+---
+source: tests/tests/serve.rs
+expression: "socket_packet.intern_and_redact(&mut redactions, ())"
+---
+body:
+  messageCursor: 1
+  messages:
+    - added: {}
+      removed:
+        - id-3
+      updated: []
+packetType: messages
+sessionId: id-1

rojo-test/serve-tests/remove_folder/default.project.json

@@ -0,0 +1,6 @@
+{
+	"name": "remove_folder",
+	"tree": {
+		"$path": "src"
+	}
+}

rojo-test/serve-tests/remove_folder/src/my-folder/test.txt

@@ -0,0 +1 @@
+This file is inside my-folder