Correctly handle deletion outside of terraform

[timofurrer]

This change fixes the read function of the following resources to properly handle out of bound deletions:

• gitlab_project
• gitlab_project_badge
• gitlab_group_badge

In case a resource is deleted outside of terraform the tf state drift and we need to remove it if the read function received a 404.

@armsnyder any ideas for how to test this in a non-hacky way?

[armsnyder]

lgtm. For testing, each TestStep has a PreConfig field you can use to run code like deleting the resource upstream.

[timofurrer]

For testing, each TestStep has a PreConfig field you can use to run code like deleting the resource upstream.

Ah yes, right! I think I'll address this later in #205