Refactor validation of jira imports

What does this MR do?

This MR extracts the validations common for Gitlab::JiraImport::BaseImporter and JiraImport::StartImportService. It also refactors specs of StartImportService so that we check the errors in correct order

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.