Exit on failure in scripts, unless user asks for it not to

[ivucica]

The user should not be given an unusable GNUstep script without an error, unless they explicitly want to ignore the errors (--keep-going or similar).

The scripts should all fail if a step fails in an unrecoverable way, so that the root script can also fail when unrecoverable.

This needs to be done carefully, to avoid bashisms when scripts are not run on a bash (or a modern version of it), since currently tools-scripts supports many environments / OSes. So, a simple set -e might not do the trick.