fcb: start using errno codes

Switch form using privater FCB error codes to
errno codes. FCB private codes convention were compatible
with <errno.h> codes:
- 0 mean success
- negative values mean errors
- similar error types.
There was no sense to kept private FCB error codes.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>