"plugin already loaded" should be a Warning, not an Error

When there are multiple requests to load the same plugin, e.g. both a "plugin_load_add" in the config file and an entry in the mysql.plugin table via "INSTALL PLUGIN", an error message is logged, e.g.:

[ERROR] mariadbd: Plugin 'unix_socket' is already installed.

IMHO this should only be a warning, not an error, as initial loading of the plugin was successful, so no real damage is done by the 2nd loading attempt in most cases.

The exception would be an attempt to load a different version of the same plugin library, but even then a warning should be sufficient IMHO, especially as even the current error output does not terminate server startup.

So both a warning and the current error would go unnoticed if a user doesn't bother to check the error log output, but the current error messages makes the situation sound much more critical than it actually is.