UBIFS: improve error messages

Ricard complaints that the following error message is odd:

"UBIFS error (pid 1578): validate_sb: bad superblock, error 8"

and he is right. This patch improves the error messages a bit and makes
them more user-friendly.

Reported-by: Ricard Wanderlof <ricard.wanderlof@axis.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>