Better error message needed for non-global setup

Attempting to setup git-hub outside a git repository needs the `--global` option, but the error message does not say so explicitly.

terminal output:
```
$> git hub setup
GitHub username [gautam]: gautam-kotian-sociomantic
GitHub password (will not be stored): 
Error: git config hub.username gautam-kotian-sociomantic failed (return code: 255)
error: could not lock config file .git/config: No such file or directory
$>
```
Shouldn't something like "current directory is not a git repo" be mentioned?