Add some logging for modelconfig problems

[chrsmith]

I spent some time trying to troubleshoot why I wasn't getting the expected results from modelconfig. And it turns out, that there are some codepaths where conf.GetCompletionsConfig(siteConfig) returns nil but without any other sort of notification.

e.g. if you are using azure-openai and do not specify the endpoint field, computed.go will just bail rather than supply a default.

		// If no endpoint is configured, this provider is misconfigured.
		if completionsConfig.Endpoint == "" {
			return nil
		}

		// If not chat model is set, we cannot talk to Azure OpenAI. Bail.
		if completionsConfig.ChatModel == "" {
			return nil
		}

This should probably just be a site configuration error, since otherwise Cody Enterprise won't work correctly. (Now with the modelconfig, we will act as if no site configuration was supplied at all and default to just using the statically embedded models, which is fairly surprising.)

Anyways, this PR just adds some logging to make this easier to track down if it happens again. I don't know how much time we would want to spend preventing errors for the "completions" config since.

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