UBIFS: Improve error message when reading superblock failed

In addition to the error message also display the error code. I had the
problem that my malloc memory was not enough (ENOMEM), and if u-boot
had displayed the error code immediately that would have saved me some
debugging.

Signed-off-by: Bernhard Walle <walle@corscience.de>

Use ubifs_err instead of printf.
Add "errno=%d" in output as suggested by Albert Aribaud.

Signed-off-by: Thomas Weber <weber@corscience.de>