Transfer failure message is not clear enough

Problem to solve

Transfer failed, please contact an admin. error message on transfer is not clear enough.

Further details

We have had a case where a user was not able to transfer a project and all he was getting was the given error message.

The user, according to upstream docs, required the correct permissions, which he had, but the destination namespace had settings blocking the creation of any sub-project:

Permissions, LFS, 2FA -> Allowed to create projects -> "No one"

We discovered this issue by looking at the code, but the error message should indicate a little bit more why the failure appeared.

To make it even worse, if an admin tries to transfer it from the project admin view, there is no message at all, it just fails with the user knowing it.

Proposal

Raise different Exception/Error messages depending on which condition failed on: https://gitlab.com/gitlab-org/gitlab-ce/blob/ad47f2094bf61867d2b5e4c24540c7c5b8a6358d/app/services/projects/transfer_service.rb#L82