Update file exists checkpointing error messages to be more helpful

[irenedea]

What does this PR do?

The parameter in Trainer is called save_overwrite, so I'm assuming these error messages should be updated to reflect that. There are no other usages of allow_overwrite as parameters, variables, or error messages.

I ran into this issue because I launched a run via foundry/mcli yaml, received one of these error messages, set add allow_overwrite: true, and my run still failed with a warning that allow_overwrite was unused 😄

What issue(s) does this change relate to?

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[dakinggg]

The parameter name to the RemoteUploaderDownloader.upload_file is actually just overwrite. So maybe the error message there should say "overwrite" but maybe add an extra note that the top level Trainer arg is called save_overwrite?

[irenedea]

@dakinggg Added a better error message at checkpoint uploading and renamed the checkpoint saver param to match the top level trainer param.

[mvpatel2000]

@irenedea is this PR still supposed to be worked on or will it be closed

[irenedea]

@mvpatel2000 making some updates right now.

[mvpatel2000]

@irenedea I think you are missing a raise ... from e here. As is, this will discard the stack trace. Can you open a new PR adding in the trace?