Cerebras exception correction and Gemini installation advice

[marklysze]

Why are these changes needed?

1. Currently, if there's an exception raised in the client classes it will fail as the Cerebras exceptions aren't defaulted to an Exception object if Cerebras is not installed. This corrects that.
2. To run the Gemini client class both google-generativeai and vertexai packages need to be installed. Currently the ImportError message omits the vertexai requirement. This corrects that.

Checks

• I've included any doc changes needed for https://autogen-ai.github.io/autogen/. See https://autogen-ai.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[marklysze]

Thanks for the review @Hk669, @henrytwo just a note that the update isn't Cerebras functionality as such, just the way we handle errors.

[Hk669]

Thanks for the review @Hk669, @henrytwo just a note that the update isn't Cerebras functionality as such, just the way we handle errors.

Yes, i just wanted to check with @henrytwo if there are any exceptions handled or provided with the cerebras library. Looks good to me. thanks @marklysze for the PR..

[henrytwo]

Thanks for the review @Hk669, @henrytwo just a note that the update isn't Cerebras functionality as such, just the way we handle errors.

Yes, i just wanted to check with @henrytwo if there are any exceptions handled or provided with the cerebras library. Looks good to me. thanks @marklysze for the PR..

We have a list of exceptions our SDK could emit here: https://github.com/Cerebras/cerebras-cloud-sdk-python?tab=readme-ov-file#handling-errors

https://github.com/autogen-ai/autogen/blob/84066b15b031430b0a817ef8c11ea415c30e04e4/autogen/oai/client.py#L52-L56

But it doesn't look like all of them are listed here. I didn't think about other exceptions we could emit. Let me know if you would like these other exceptions to be handled the same way as you did in this PR too.