Add a debug check for HTML tag handlers being available.

This helps to detect the situation when HTML tag handlers are not linked in at
all, as it can happen with MSVC when using static libraries, as this doesn't
prevent wxHTML from parsing HTML, it just doesn't render it correctly at all
and it can be difficult to understand why exactly does this happen, so try to
detect this situation and provide a hint.

src/html/htmlpars.cpp

@@ -316,6 +316,13 @@ void wxHtmlParser::AddTag(const wxHtmlTag& tag)
         if (m_stopParsing)
             return;
     }
+#if wxDEBUG_LEVEL
+    else if (m_HandlersHash.empty())
+    {
+        wxFAIL_MSG( "No HTML tag handlers registered, is your program linked "
+                    "correctly (you might need to use FORCE_WXHTML_MODULES)?" );
+    }
+#endif // wxDEBUG_LEVEL
     if (!inner)
     {
         if (tag.HasEnding())