systemc: Change an error message to match the reference outputs.

This is different from the message the Accellera implementation would
print, so it looks like it wouldn't pass this test as is.

Change-Id: I0286fd3e3df7bc3e87f38c35086a32d8dec0bcc9
Reviewed-on: https://gem5-review.googlesource.com/c/12814
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
3 files changed