kcoreaddons_add_plugin: Throw error when we have unparsed args
In case one forgets to add the SOURCES option before passing in the files,
we would simply not add them to the library and don't emit any kind of warning.
In case of static plugins we would get a build error, but for dynamic plugins we would only get a semi-useful error from QPluginLoader that no plugin/metadata could be found.
Considering that the unexpected argument can be a long list of arguments, I don't think it is worth printing it out. Consumers should check out the docs for the usage information.