Provide better error description when migration fails to start due to backend validations

What does this MR do?

Related issue: #300684 (closed)

This MR implements showing error provided by backend on frontend

No changelog since feature is behind feature flag

Testing guidelines

• Enable feature bulk_import via Feature.enable(:bulk_import)
• Open "New group" (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)
• Make sure you have flightjs group on your instance (is shipped with default GDK installation)
• Try to import flightjs group to flightjs / flightjs
• See error

Video

New_Group___GitLab__7_

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.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done