grub_fs_probe(): dprint errors from filesystems

When filesystem detection fails, all that's currently debug-logged is a
series of messages like:

    grub-core/kern/fs.c:56:fs: Detecting ntfs...
    grub-core/kern/fs.c:76:fs: ntfs detection failed.

repeated for each filesystem.  Any messages provided to grub_error() by
the filesystem are lost, and one has to break out gdb to figure out what
went wrong.

With this change, one instead sees:

    grub-core/kern/fs.c:56:fs: Detecting fat...
    grub-core/osdep/hostdisk.c:357:hostdisk: reusing open device
    `/path/to/device'
    grub-core/kern/fs.c:77:fs: error: invalid modification timestamp for /.
    grub-core/kern/fs.c:79:fs: fat detection failed.

in the debug prints.

Signed-off-by: Robbie Harwood <[email protected]>
(cherry picked from commit 838c79d658797d0662ee7f9e033e38ee88059e02)

grub-core/kern/fs.c

@@ -74,6 +74,7 @@ grub_fs_probe (grub_device_t device)
 	  if (grub_errno == GRUB_ERR_NONE)
 	    return p;
 
+	  grub_dprintf ("fs", _("error: %s.\n"), grub_errmsg);
 	  grub_error_push ();
 	  grub_dprintf ("fs", "%s detection failed.\n", p->name);
 	  grub_error_pop ();