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/FileSystem.h

@@ -31,7 +31,7 @@ public:
     static void sync();
     static void lock_all();
 
-    virtual bool initialize() = 0;
+    virtual KResult initialize() = 0;
     virtual StringView class_name() const = 0;
     virtual Inode& root_inode() = 0;
     virtual bool supports_watchers() const { return false; }