backend: Add configurable non fatal error list

backend: Add configurable non fatal error list

Sometimes it is reasonable to perform test nearly system limits where
errors are possible and expected, in that case one may provide non
fatal error list which will be ignored during execution.
This patch add two options:
    ignore_error: List of non fatal error
    error_dump:   Whenever ignored list should be dumped to log.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>