Simplification of graphql mutations

What does this MR do?

This MR simplifies few graphql mutations in 2 ways:

1. It uses a method errors of ServiceResponse which was introduced in !33501 (merged)
2. For JiraImport::Start it simplifies fetching project and its validation. We don't need to check project_path and then project presence as it is handled in authorized_find! (we test this in the related specs, no changes needed there, still passing)

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.