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 
Do we need to say, try again, using --force?