pty: downgrade error to debug message on I/O errors

I/O errors aren't fatal or important on PTYs so ignore them and mark it as
debug message.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

src/pty.c

@@ -407,8 +407,8 @@ static int read_buf(struct kmscon_pty *pty)
 				  pty->child);
 			break;
 		} else if (errno != EWOULDBLOCK) {
-			log_err("cannot read from pty of child %d (%d): %m",
-				pty->child, errno);
+			log_debug("cannot read from pty of child %d (%d): %m",
+				  pty->child, errno);
 			break;
 		}
 	} while (len > 0 && --num);