warn if v2_key exists with meaningful error

fix_auth (the most common place to generate a key) tries to provide
a user context that they should not just remove the file.

The cli is more powerful, so a --force option is provided

- cli only replaces v2_key if --force
- cli does replace v2_key if fetching from remote
- fix_auth displays warning. User must backup key first

/cc @Fryguy @jrafanie 
We need to backup this key. But adding that to the workflow complicates things

workflows: 

- no key exists (cli, fix_auth)
- key exists but we are copying from remote. Just overwrite. (cli only)
- key exists but generating. Do not overwrite (cli, fix_auth)

When you add backup to the mix, you then probably want to add `--force` to `fix_auth`. You want to add backup options per each of the above.

Seems so complex.