Don't error if fine-tune serialization dir already exists (#1622)
Don't error if fine-tune serialization dir already exists (#1622)

Changing this to match the behavior of other commands, like [`train`](https://github.com/allenai/allennlp/blob/82686c10bdca062a7e41825000c39372de354479/allennlp/commands/train.py#L222) and [`make-vocab`](https://github.com/allenai/allennlp/blob/1402b7c02aede1a8f832f64cdfb2e9ef1157faca/allennlp/commands/make_vocab.py#L83). 

This causes `fine-tune` to fail with beaker at the moment.
allenai/allennlp,allenai/allennlp,allenai/allennlp,allenai/allennlp