This commit completely rewrites keep-going mode. Instead of relying
solely on "set -e", which has some subtle limitations (such as being
off anywhere inside a conditional), use an ERR trap to record errors.
Run each component in a subshell. This way a component can set
environment variables, change the current directory, etc., without
affecting other components.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>