Running a command without login could have a more meaningful error

Just for fun, I executed `keybase sign -m 'Hello'` before `keybase login` after installing the client on Ubuntu 14.04. It failed (I'm assuming because I wasn't logged in and so had no default identity?) with the error:

```
$ keybase sign -m 'Hello'
warn: No config file found; tried '/home/mnem/.keybase/config.json'
error: missing or invalid input (error #100)
```

Start at the entry point for the `keybase sign` command and reproduce the command before `keybase login` on Ubuntu. Compare the current “missing or invalid input” output with the login-related failure state; done means the command reports a meaningful cause when no identity is available.

Written by the indexing model from the issue text.