Don't warn, just debug about not loadable plugin + Fix skip already loaded plugin

• The first patch seems to fix a missing unload
• The second suppress these "kf.sonnet.core: Sonnet: Unable to load plugin ..." message
• The third revert the first and change the order of check/load plugin

BTW: I think when these message is somehow not correct. The plugin is loaded, because it is there, but then it seems to report "nope" because it can't find the needed speller lib (which is not installed).

A reasonable way might be to point out uninstalled spellers in Sonnet's configuration dialog. And perhaps let the user choose which ones to load (or not, for whatever reason) even if they are installed. Currently is tried to use something that the user has not installed, maybe intentionally or not. This warning make more noise (complains) than is sensible.