Better error for misconfigured git submodule

Steps to reproduce

1. mkdir tmp
2. cd tmp
3. git init
4. echo 'github "antitypical/Result"'>Cartfile
5. carthage update --use-submodules --no-build
6. git add .
7. git commit -m 'Initial'
8. git rm Carthage/Checkouts/Result
9. git reset HEAD .gitmodules
10. git checkout -- .gitmodules
11. git commit -m 'Bork it'
12. carthage bootstrap --use-submodules --no-build

Expected A nice error that explains what you do and how to fix it or possibly even fix it automatically.

Fix:

1. git submodule add -f {url to submodule} {path to submodule}
2. git commit

Actual This unfriendly error. If you don't know to avoid doing the above, you definitely won't understand the error.

Parse error: expected submodule commit SHA in output of task (ls-tree -z HEAD Carthage/Checkouts/Result) but encountered: