parsetl: flush the errors

This fixes an issue in the on-line translator, where error message would
not be output in the correct <div>.

* spot/parsetl/fmterror.cc (format_parse_errors): Flush the stream.

spot/parsetl/fmterror.cc

@@ -89,6 +89,7 @@ namespace spot
           os << '\n' << it.second << "\n\n";
           printed = true;
         }
+      os.flush();
       return printed;
     }
   }