CMP0037: Restore diagnostic message for invalid ALIAS target names

Refactoring in commit 7a4c02cb38 (cmGlobalGenerator: factor out
messaging for CMP0037, 2023-09-24, v3.28.0-rc1~39^2~7) incorrectly
switched to reporting the aliased target name instead of the invalid
name of the alias itself.

Fixes: #25979