Prompt for 2fa on stderr instead of stdout

This means that a 2fa prompt is still visible if, e.g., one is piping the result of a heroku command which requires 2fa into another command:

Before this patch:

```console
maciek@gamera:~/code/heroku$ bin/heroku config -a db-corral | grep foo
# hangs indefinitely after the above; it does actually work if i just feed it
# my 2fa code blind, but i have to know to do that
ccccccdlnndrcidhrtgligivrvvlfetgdnkukhrrhfef 
foo:                               4
test:                              foo
maciek@gamera:~/code/heroku$
```

After this patch:

```console
maciek@gamera:~/code/heroku$ bin/heroku config -a db-corral | grep foo
# now shows prompt before waiting for 2fa code
Two-factor code: ccccccdlnndrtrnebkkclicdbbfcrhiideekjgncetun
foo:                               4
test:                              foo
maciek@gamera:~/code/heroku$
```