Issue 2099: have Execute() print an error message if an action

Issue 2099:  have Execute() print an error message if an action
fails.  Better document the behavior of returning the exit status,
and that exit-on-failure is the SConscript writer's responsibility.

git-svn-id: http://scons.tigris.org/svn/scons/trunk@3310 fdb21ef1-2011-0410-befe-b5e4ea1792b1