LibIPC+IPCCompiler+AK: Make IPC value decoders return ErrorOr<void>

This allows us to use TRY() in decoding helpers, leading to a nice
reduction in line count.

Meta/Lagom/Tools/CodeGenerators/IPCCompiler/main.cpp

@@ -386,7 +386,7 @@ public:
 
                 parameter_generator.append(R"~~~(
         @parameter.type@ @parameter.name@ = @parameter.initial_value@;
-        if (!decoder.decode(@parameter.name@))
+        if (decoder.decode(@parameter.name@).is_error())
             return {};
 )~~~");