Update of repository with sub-modules silently remove submodule if no commit found

[stchar]

Description:
I had had migrated a Mercurial repository with submodules to git. Then team pushed new changes to the Mercurial so I had to re-import it to git. Unfortunately I did a mistake: I forgot to re-import changes in submodule repository and just re-imported the parent repo. After a verification of git commits I found out that submodule was deleted.

Steps to reproduce

1. create submodule mapping
   "submodule"="../submodule"

2. Import submodule

3. Import parent repository

4. Add new commit in submodule mercurial repository

5. Add new commit in parent mercurial repository

6. Run hg-fast-import for parent repository only

Result:
Parent repository contains a commit with removal of sub-module.
Stdout contains a warning that commit of sub-module is missing

Expected result:
hg-fast-import.sh should throw error. Migration should be halted.

[frej]

That sounds like a bug, the reasonable thing is, as you say, to error out. @MokhamedDakhraui, @johannesc can you take a look?