Functions in catalog should report better error message when functionConfig is missing

[natasha41575]

See test case fn-render/no-fnconfig.

The error message for set-label is failed to configure function: function config must be a ConfigMap or SetLabelConfig. This can be improved, e.g. missing functionConfig, must specify ConfigMap or SetLabelConfig.

All functions should have a good error message for a missing functionConfig.

cc @droot @mengqiy @Shell32-Natsu

[frankfarzan]

Also, note functionConfig is one word: https://kpt.dev/book/02-concepts/03-functions