c++/modules: Clarify error message in read_enum_def

This error message reads to me the wrong way around, particularly in the
context of other errors.  Updated so that the ellipsis connect.

gcc/cp/ChangeLog:

	* module.cc (trees_in::read_enum_def): Clarify error.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/enum-bad-1_b.C: Update error message.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>