UBIFS: small amendments in the LEB scanning code

This patch fixes few minor things I've spotted while going through
code:

1. Better document return codes
2. If 'ubifs_scan_a_node()' returns some thing we do not expect,
   treat this as an error.
3. Try to do recovery only when 'ubifs_scan()' returns %-EUCLEAN,
   not on any error.
4. If empty space starts at a non-aligned address, print a message.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Adrian Hunter <Adrian.Hunter@nokia.com>