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>
diff --git a/verify.c b/verify.c
index 5a94281..91a9077 100644
--- a/verify.c
+++ b/verify.c
@@ -1033,7 +1033,7 @@
 			put_io_u(td, io_u);
 			if (!ret)
 				continue;
-			if (td->o.continue_on_error &&
+			if (td->o.continue_on_error & ERROR_TYPE_VERIFY &&
 			    td_non_fatal_error(ret)) {
 				update_error_count(td, ret);
 				td_clear_error(td);