Added error handling

include/cmd/cmd_tr.hpp

@@ -33,7 +33,7 @@ namespace fsh {
       protected:
 
         std::string replace_all(std::string str,const std::string &what,const std::string &with) {
-            if(what == "") throw std::runtime_error("\"\" cannot be used as what");
+            if(what == "") throw util::CmdError("What cannot be empty string");
 
             unsigned long long pos = str.find(what);