hw/mips: load_elf_strerror to report kernel loading failure

Emulated MIPS boards bail out with a simple "could not load kernel" when
a kernel could not be load, without specifying the underlying reason.
Fix that by calling load_elf_strerror.

At the same time use error_report to report the error instead of
fprintf.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>