* kern/err.c (grub_error_push): Added new function to support error
stacks.
(grub_error_pop): Likewise.
(grub_error_stack_items): New local variable to support error stacks.
(grub_error_stack_pos): Likewise.
(grub_error_stack_assert): Likewise.
(GRUB_ERROR_STACK_SIZE): Added new define to configure maximum error
stack depth.
(grub_print_error): Added support to print errors from error stack.
* include/grub/err.h (grub_error_push): Added function prototype.
(grub_error_pop): Likewise.