fix: change CloudStorageFileSystemProvider to throw a FileAlreadyExistsException if copy receives a 412

[BenWhitehead]

Fixes #762

[sydney-munro]

LGTM. It would be nice if we could get the service team to send out a more helpful error message rather than us making the assumption. Have we put out that feature request?

[BenWhitehead]

In this case there's not something the backend can really do for us. If StandardCopyOption.REPLACE_EXISTING isn't present when the method is called, the FileSystemProvider will explicitly set an expected target generation of 0 (which means expect target to not already exist). If the target already exists, the backend returns 412 to let us know our precondition constraint hasn't been met at which point we can then determine what that means contextually for us. In this case, we would overwrite a "file" we weren't instructed to.