GenAI utils: Completion hook improvements

[lmolkova]

1. Can we log a warning when a name is passed into the envvar but no match is found in the entry point?
2. When a loaded object does not implement CompletionHook, let's use logger.warning instead of debug since this seems like a real configuration bug.
3. When an exception is caught, the log says the noop is being used by the iteration continues for the rest of the entry points (technically would be ok unless duplicate names).

Originally posted by @lzchen in #4315 (comment)