IncompleteScanError class

What does this MR do?

Add IncompleteScanError class.

What are the relevant issue numbers?

gitlab-org/gitlab#386070 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for this project, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Security reports checked/validated by reviewer