input/vcd: raise severity of messages about unsupported data types

The VCD input module did check for unsupported data types in $var
declarations. But the emitted message only was of INFO level and thus
would not be seen at the default log level. Use ERR instead to raise
awareness. Let users see by default that the input contains data which
cannot get processed (and why the import terminated prematurely).

This addresses part of bug #1757.