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/clean/seabios

@@ -21,16 +21,18 @@
 [ "x${DEBUG+set}" = 'xset' ] && set -v
 set -u -e
 
+. "include/err.sh"
+
 # clean bucts
 # --------------------------------------------------------
 
 printf "Cleaning the previous builds of seabios\n"
 
-rm -Rf elf/seabios || exit 1
+[ ! -d "elf/seabios" ] || rm -Rf elf/seabios || err "cannot remove elf/seabios"
 [ ! -d "seabios/" ] && exit 0
 
 for x in seabios/*; do
 	[ "${x}" = "seabios/seabios" ] && continue
 	[ ! -d "${x}" ] && continue
-	make -C "${x}" distclean || exit 1
+	make -C "${x}" distclean || err "cannot distclean tree, ${x}"
 done