resolve #153: git ship errors with empty commit message

@kevgo 
What about this:

```cucumber
And I run `git ship` and enter an empty commit message
```

the "while allowing errors" part was only necessary to distinguish it from the similar sounding case where there should be no errors. This is a clearly an error, and we are checking for it on the next line. So I think the "while allowing errors" is a bit redundant here.