Improve zfs.sh error messages

Ensure an error message is logged when the 'zfs.sh' script fails
to either load a module or if udev fails to create the /dev/zfs
device.  Error messages for missing KERNEL_MODULES are suppressed
because that functionality may just be built-in to the kernel.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

scripts/common.sh.in

@@ -207,7 +207,11 @@ load_module() {
 		echo "Loading ${NAME} ($@)"
 	fi
 
-	${LDMOD} $* &>/dev/null || ERROR="Failed to load $1" return 1
+	${LDMOD} $* &>/dev/null
+	if [ $? -ne 0 ]; then
+		echo "Failed to load ${NAME} ($@)"
+		return 1
+	fi
 
 	return 0
 }
@@ -216,7 +220,7 @@ load_modules() {
 	mkdir -p /etc/zfs
 
 	for MOD in ${KERNEL_MODULES[*]}; do
-		load_module ${MOD}
+		load_module ${MOD} >/dev/null
 	done
 
 	for MOD in ${MODULES[*]}; do

scripts/zfs.sh

@@ -72,8 +72,8 @@ if [ ${UNLOAD} ]; then
 else
 	stack_clear
 	check_modules || die "${ERROR}"
-	load_modules "$@"
+	load_modules "$@" || die "Failed to load modules"
-	wait_udev /dev/zfs 30
+	wait_udev /dev/zfs 30 || die "'/dev/zfs' was not created"
 fi
 
 exit 0