LibGUI: Make GUI::try_create_default_icon() tolerate single-size icons

Some icons only exist in 16x16 and we should still allow them to be
loaded via the LibGUI default icon system.

This patch also reimplements GUI::default_icon() as a MUST() wrapper
around try_create_default_icon().

Userland/Libraries/LibGUI/Icon.cpp

@@ -74,6 +74,11 @@ void IconImpl::set_bitmap_for_size(int size, RefPtr<Gfx::Bitmap>&& bitmap)
 }
 
 Icon Icon::default_icon(StringView name)
+{
+    return MUST(try_create_default_icon(name));
+}
+
+ErrorOr<Icon> Icon::try_create_default_icon(StringView name)
 {
     RefPtr<Gfx::Bitmap> bitmap16;
     RefPtr<Gfx::Bitmap> bitmap32;
@@ -81,13 +86,9 @@ Icon Icon::default_icon(StringView name)
         bitmap16 = bitmap_or_error.release_value();
     if (auto bitmap_or_error = Gfx::Bitmap::try_load_from_file(String::formatted("/res/icons/32x32/{}.png", name)); !bitmap_or_error.is_error())
         bitmap32 = bitmap_or_error.release_value();
-    return Icon(move(bitmap16), move(bitmap32));
-}
 
-ErrorOr<Icon> Icon::try_create_default_icon(StringView name)
-{
-    RefPtr<Gfx::Bitmap> bitmap16 = TRY(Gfx::Bitmap::try_load_from_file(String::formatted("/res/icons/16x16/{}.png", name)));
-    RefPtr<Gfx::Bitmap> bitmap32 = TRY(Gfx::Bitmap::try_load_from_file(String::formatted("/res/icons/32x32/{}.png", name)));
+    if (!bitmap16 && !bitmap32)
+        return Error::from_string_literal("Default icon not found"sv);
 
     return Icon(move(bitmap16), move(bitmap32));
 }