Group Migration: Ensure group validation produce correct error message when name is already taken by project

What does this MR do?

This MR ensures that correct "Name already exists" message is reported when target group location is occupied by project

Video

Screencast_from_06-02-2021_07_44_25_PM

Related to #330885 (closed)

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)
• Set for any group existing project group & path as target
• Ensure that you see error message

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)

  • No changelog as feature is behind feature flag

• I have added/updated documentation, or it's not needed. (Is documentation required?)

• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)

• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)

• I have self-reviewed this MR per code review guidelines.

• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)

• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.