scons: Add "warning" and "error" methods.

scons: Add "warning" and "error" methods.

These methods will make reporting errors less verbose and more
consistent, since they'll handle some formating, setting colors,
prefixing with an appropriate "Warning:" or "Error:" tag, and exiting
in the case of an error.

Change-Id: Iddea5bf342a4fc4b26002d8e98292f9dc57fa8cc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22885
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>