finalization-test: RELEASE_HIDDEN_API_EXPORTABLE_STUBS: better error message

Improve the error message in case RELEASE_HIDDEN_API_EXPORTABLE_STUBS is
not set to 'true' on 'next'.

Test: build-flag --quiet --release=next set RELEASE_HIDDEN_API_EXPORTABLE_STUBS false && atest finalization-test
Change-Id: Ib10d90f6234f3b97ccdda7ec7cc32d1790c8f616