write_graphite plugin: config should fail on error

This is the preferred way of handling config, as it can be seen in snmp.c. It's
important to fail on a config error to simplify code in other callbacks.
write_graphite plugin: avoid flooding the log with the protocol error check

Moving the protocol error checking from `wg_callback_init` to `wg_config_node`.
This avoids flooding the log, as suggested by Florian Forster (@octo).