up2k-js: fix exfat's http502 allergy;

normally, to avoid excessive handshaking, unexpected server errors are
handled by continuing onto the next chunk and dealing with it at eof

this doesn't work on filesystems that dont support sparse files, so
discard the chunk-queue and re-handshake to keep it sequential

copyparty/web/up2k.js

@@ -3008,7 +3008,7 @@ function up2k_init(subtle) {
             }
             else {
                 xhrchk(xhr, L.u_cuerr2.format(snpart, Math.ceil(t.size / chunksize), esc(t.name)), "404, target folder not found (???)", "warn", t);
-                if (txt.indexOf('unknown wark') == 0) {
+                if (!t.sprs || txt.indexOf('unknown wark') == 0) {
                     t.cooldown = t.coolmul = 0;
                     unqueue_up(t);
                 }