scons: Use the new error() and warning() methods.

Also clean up some error messages which were missing capitalization,
etc.

Change-Id: Iaef6b4343a693d30b579e72218cbb7723ebf7d48
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22886
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed