src/plugin.c: plugin_dispatch_values: Improve the error messages.

src/plugin.c: plugin_dispatch_values: Improve the error messages.

If no write plugin was loaded or no DS was defined, the
`plugin_dispatch_values' used to fail silently. This is of course not good, so
this commit introduces error messages for this case, so that users can see why
their setup is not working properly.

Also, the ``no such DS type'' message was promoted from DEBUG to INFO.