grub-mkstandalone: reduce sharp edges

It is a major foot gun that grub-mkstandalone silently drops files.
As a first step, we can improve documentation and add warn messages.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

util/grub-mkstandalone.c

@@ -112,7 +112,7 @@ argp_parser (int key, char *arg, struct argp_state *state)
 
 struct argp argp = {
   options, argp_parser, N_("[OPTION] SOURCE..."),
-  N_("Generate a standalone image (containing all modules) in the selected format")"\v"N_("Graft point syntax (E.g. /boot/grub/grub.cfg=./grub.cfg) is accepted"), 
+  N_("Generate a standalone image (containing all modules) in the selected format")"\v"N_("Graft point syntax (E.g. /boot/grub/grub.cfg=./grub.cfg) is accepted. The path on the right must point to a regular file."),
   NULL, help_filter, NULL
 };
 
@@ -189,8 +189,10 @@ add_tar_file (const char *from,
 
   COMPILE_TIME_ASSERT (sizeof (hd) == 512);
 
-  if (grub_util_is_special_file (from))
-    return;
+  if (grub_util_is_special_file (from)) {
+      grub_util_warn (_("skip: %s is not a regular file"), from);
+      return;
+  }
 
   mtime = grub_util_get_mtime (from);
 
@@ -273,7 +275,7 @@ add_tar_file (const char *from,
   compute_checksum (&hd);
 
   fwrite (&hd, 1, sizeof (hd), memdisk);
- 
+
   while (1)
     {
       r = grub_util_fd_read (in, grub_install_copy_buffer, GRUB_INSTALL_COPY_BUFFER_SIZE);