Distinguish between success and an invalid mod in flash-image's return value
Remove multimod example

It's inefficient and redundant now that we have support for the
flashcast-mods directory.
Remove shebang lines from sample imager.sh files

They only cause confusion since executing the scripts directly
(instead of through flash-image) will fail.
Add "set -e" to the beginning of sample mods

In most cases, failing immediately if a command fails is the correct
thing to do.