[libc++][format] Improve an exception message.

The fix in D116381 makes an existing exception message wrong. This
improves the message and fixes the associated unit tests.

Note other message can be also be improved, but that will be done later.
Changing these messages may cause merge conflicts with other patches
that are under review or WIP.

Depends on D116381

Reviewed By: #libc, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D116495