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-key option is provided

- cli only replaces v2_key if --force-key
- cli does replace v2_key if fetching from remote
- fix_auth displays warning. User must backup key first
- cli displays warning. offers --force-key option

https://bugzilla.redhat.com/show_bug.cgi?id=1153777

/cc @Fryguy @jrafanie 
You can just call `raise` with no parameters to reraise the original exception.