Expand continue_on_error to select which type of error to allow

Expand continue_on_error to select which type of error to allow

This expands the continue_on_error option to take a string specifying
what type of error to continue on, breaking out errors into read,
write, and verify.  (Sync, trim, and anything else not specifically a
read are considered write operations for the sake of error
continuation.)

Backwards compatibility is retained by allowing =0 and =1 values to
specify none and all, respectively.

Signed-off-by: Jens Axboe <axboe@kernel.dk>