Update wording for group transfer path error

What does this MR do and why?

Before transfering a group, it's checked if there is already Namespace with the same path. Currently this effectively checks only if there is a subgroup with the same path. If there is a project with the same path, transfer fails too, but in the next step because updated group validation fails with error "Validation failed: Group URL has already been taken".

When we create/backfill project namespace for each project, we will effectively check both for subgroup or project path conflicts (because project namespace will be created with the same path as project). So we can just update the error message to mention that there is either group or project with the same path.

Related to #341823 (closed)

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