rows.js: Add error handling to rows.id().

We get random blueslip errors from code that
calls rows.id(), and the error messages are
rarely helpful.

frontend_tests/node_tests/lightbox.js

@@ -53,6 +53,8 @@ run_test('youtube', () => {
     const link = $.create('link-stub');
     const msg = $.create('msg-stub');
 
+    msg.attr("zid", "4321");
+
     $(img).attr('src', href);
 
     $(img).closest = (sel) => {

static/js/rows.js

@@ -42,7 +42,24 @@ exports.last_visible = function () {
 };
 
 exports.id = function (message_row) {
-    return parseFloat(message_row.attr('zid'));
+    /*
+        For blueslip errors, don't return early, since
+        we may have some code now that actually relies
+        on the NaN behavior here.  We can try to clean
+        that up in the future, but we mainly just want
+        more data now.
+    */
+    if (message_row.length !== 1) {
+        blueslip.error("Caller should pass in a single row.");
+    }
+
+    const zid = message_row.attr('zid');
+
+    if (zid === undefined) {
+        blueslip.error("Calling code passed rows.id a row with no zid attr.");
+    }
+
+    return parseFloat(zid);
 };
 
 const valid_table_names = {