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/DevFS.cpp

@@ -56,9 +56,9 @@ DevFS::~DevFS()
 {
 }
 
-bool DevFS::initialize()
+KResult DevFS::initialize()
 {
-    return true;
+    return KSuccess;
 }
 
 Inode& DevFS::root_inode()