Improve modify_override errors, fix no NVT case (bp #1435)

[mergify]

This is an automatic backport of pull request #1435 done by Mergify.

Cherry-pick of 99918c4 has failed:

On branch mergify/bp/master/pr-1435
Your branch is up to date with 'origin/master'.

You are currently cherry-picking commit 99918c446.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/gmp.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/manage_sql.c

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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