Print a warning in case a fallback value wasn't found

components/fallback/fallback.cpp

@@ -1,5 +1,7 @@
 #include "fallback.hpp"
 
+#include <iostream>
+
 #include <boost/lexical_cast.hpp>
 
 
@@ -17,6 +19,7 @@ namespace Fallback
         std::map<std::string,std::string>::const_iterator it;
         if((it = mFallbackMap.find(fall)) == mFallbackMap.end())
         {
+            std::cerr << "Warning: fallback value " << fall << " not found." << std::endl;
             return "";
         }
         return it->second;