Kernel: Make FileSystem::initialize() return KResult

This forced me to also come up with error codes for a bunch of
situations where we'd previously just panic the kernel.

Kernel/FileSystem/Plan9FileSystem.h

@@ -22,7 +22,7 @@ public:
     virtual ~Plan9FS() override;
     static NonnullRefPtr<Plan9FS> create(FileDescription&);
 
-    virtual bool initialize() override;
+    virtual KResult initialize() override;
 
     virtual bool supports_watchers() const override { return false; }