UBIFS: add ubifs_err() to print error reason

This patch adds ubifs_err() output to some error paths to tell the user
what's going on.

Artem: improve the messages, rename too long variable

Signed-off-by: Subodh Nijsure <snijsure@grid-net.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Acked-by: Brad Mouring <brad.mouring@ni.com>
Acked-by: Terry Wilcox <terry.wilcox@ni.com>
Acked-by: Gratian Crisan <gratian.crisan@ni.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>