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/SysFS.cpp

@@ -75,9 +75,9 @@ SysFS::~SysFS()
 {
 }
 
-bool SysFS::initialize()
+KResult SysFS::initialize()
 {
-    return true;
+    return KSuccess;
 }
 
 Inode& SysFS::root_inode()