cmd: ubi: Add additional message upon UBI attach error

cmd: ubi: Add additional message upon UBI attach error

When trying to attach an UBI MTD partition via "ubi part", it may happen
that the MTD partition defined in U-Boot (via mtdparts) is not big
enough than the one, where the UBI device has been created on. This
may lead to errors, which are not really descriptive to debug and
solve this issue, like:

ubi0 error: vtbl_check: too large reserved_pebs 1982, good PEBs 1020
ubi0 error: vtbl_check: volume table check failed: record 0, error 9

or:

ubi0 error: init_volumes: not enough PEBs, required 1738, available 1020
ubi0 error: ubi_wl_init: no enough physical eraseblocks (-718, need 1)
ubi0 error: ubi_attach_mtd_dev: failed to attach mtd1, error -12

Lets add an additional message upon attach failure, to aid the U-Boot
user to solve this problem.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>