build.py: Provide useful diagnostics when exiting.

Running scripts with 'set -e' produces silent failures with no
diagnostic. Add an exit handler which produces diagnostics, including
details of what was running if the shell seems to be bash.

Signed-off-by: Peter Seebach <peter.seebach@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>