aconfig: update error message in modify_parsed_flags_based_on_mode

This commit updates the error message in
modify_parsed_flags_based_on_mode, when there is no certain mode flags
in the corresponding library, and this change remove "." at the end of
the error message to conform to existing standard in this project.

Bug: 311152507
Test: atest aconfig.test
Change-Id: Ib120e7f7252981bbeb6ca1cf785da24f329507b4