scripts: never exit 1, always call err instead

this same change has been applied, selectively, to
certain return statements. the general rule is this:
the return statement should only be used to direct
logic within a script, where certain non-errors
states are used to skip certain actions; the exit
command should *never* be used to return non-zero,
except by err(). in so doing, we ensure easier
debugging of the build system

also: strip_rom_image in build/release/roms was
running "continue" when a rom file didn't exist,
despite not being a while/for loop. i make it
return (non-error condition) instead

it's ok for a script to exit 0, where appropriate,
but perhaps a function could also be written for it

Signed-off-by: Leah Rowe <leah@libreboot.org>

resources/scripts/build/grub/utils

@@ -27,7 +27,7 @@ set -u -e
 main()
 {
 	printf "Building GRUB\n"
-	[ -d "grub/" ] || ./fetch grub || exit 1
+	[ -d "grub/" ] || ./fetch grub || err "cannot fetch grub"
 	build_grub
 }