GUI::show_error() is now generalized for std::string and const char*

Fixed return type of std::string translate_utf8() with context
(incorrectly returned wxString, should return std::string).
Fixed double translation of BackgroundSlicingProcess::validate() in Plater.

src/slic3r/GUI/PrintHostDialogs.cpp

@@ -275,7 +275,7 @@ void PrintHostQueueDialog::on_error(Event &evt)
 
     on_list_select();
 
-    GUI::show_error(nullptr, std::move(errormsg));
+    GUI::show_error(nullptr, errormsg);
 }
 
 void PrintHostQueueDialog::on_cancel(Event &evt)