kernel-install: if a plugin fails, return error immediately

Since the first version in 81516adcb7,
kernel-install would "gather" a return value by summing the exit codes
of the plugins… This makes no sense, because those are not additive values.

Let's just break off immediately. We now implement cleanup via trap, so if we
break, we should leave no garbage behind.

src/kernel-install/kernel-install.in

@@ -336,7 +336,7 @@ case "$COMMAND" in
 
             err=$?
             [ $err -eq $skip_remaining ] && break
-            ret=$(( ret + err ))
+            [ $err -ne 0 ] && exit $err
         done
         ;;
 
@@ -346,7 +346,7 @@ case "$COMMAND" in
             "$f" remove "$KERNEL_VERSION" "$ENTRY_DIR_ABS"
             err=$?
             [ $err -eq $skip_remaining ] && break
-            ret=$(( ret + err ))
+            [ $err -ne 0 ] && exit $err
         done
 
         if [ "$MAKE_ENTRY_DIR_ABS" -eq 0 ]; then