Kernel: Make Kernel::VMObject allocation functions return KResultOr

This makes for nicer handling of errors compared to checking whether a
RefPtr is null. Additionally, this will give way to return different
types of errors in the future.

Kernel/Graphics/Intel/NativeGraphicsAdapter.cpp

@@ -639,6 +639,7 @@ void IntelNativeGraphicsAdapter::initialize_framebuffer_devices()
     VERIFY(m_framebuffer_height != 0);
     VERIFY(m_framebuffer_width != 0);
     m_framebuffer_device = FramebufferDevice::create(*this, 0, address, m_framebuffer_width, m_framebuffer_height, m_framebuffer_pitch);
-    m_framebuffer_device->initialize();
+    // FIXME: Would be nice to be able to return a KResult here.
+    VERIFY(!m_framebuffer_device->initialize().is_error());
 }
 }