Improve error reporting when fetching unexisting commit

[Hyask]

When fetching a commit in a submodule that didn't exist, the following log was printed:

Command failed due to the following error(s):
Error: Error while checkouting quasar
  Caused by: Could not fetch submodule
  Caused by: early EOF; class=Net (12); code=Eof (-20)
Error: Command terminated with errors

while git submodule update --init reported:

fatal: Fetched in submodule path 'quasar', but it did not contain b93976279dcd10ae10889b2f3b61abb98e07a88a. Direct fetching of that commit failed.