WebP: Log error if there is one

Else, WebP files with a broken header just return "Decoding failed"
without any more details. This way, there's some debug logging with
more details.

Maybe we'll want to remove this again since it might lead to duplicate
error messages for files that have their error not in the header.
We'll see how this feels. (But most files don't have any errors, so
it's probably fine.)

Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp

@@ -630,6 +630,7 @@ WebPImageDecoderPlugin::~WebPImageDecoderPlugin() = default;
 bool WebPImageDecoderPlugin::set_error(ErrorOr<void> const& error_or)
 {
     if (error_or.is_error()) {
+        dbgln("WebPLoadingContext error: {}", error_or.error());
         m_context->state = WebPLoadingContext::State::Error;
         return true;
     }