nandwrite: fix error handling

nandwrite: fix error handling

Artem Bityutskiy wrote:
> Yes, write and erase failure mean that the erasblock is bad. But I think
> marking a block as bad straight away is just dangerous. Who knows may be
> this is a small glitch in a bus, or a software bug, or some-one
> corrupted driver's memory, or whatever. This is why UBI is doing
> eraseblock torturing before marking it as bad. And it is very careful
> about error codes - only EIO code is considered as a reason to mark an
> eraseblock as bad.

Fixed broken behavior in case of write failure. More specifically:
- Only try to mark a block bad if the errors are EIO. Other errors
will abort the tool.
- Also abort the tool if the marking fails instead of ignoring it.

Signed-off-by: Jehan Bing <jehan@orb.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>