reorder tablet checks in vtgate tablet gateway

[austenLacy]

Description

The error message returned from the tablet gateway is misleading.

I swapped the tablet checks to check if the primary is serving before checking the resharding state.

For example, during a recent incident where a primary mysql was down Vitess was returning current keyspace is being resharded error messages back to an application which is very confusing. If we look at the tablet serving state however we can see that the primary is NOT_SERVING.

ro@cluster(replica)> show vitess_tablets;
+-------------+----------------------------------+-------+------------+-------------+------------------------+---------------+----------------------+
| Cell        | Keyspace                         | Shard | TabletType | State       | Alias                  | Hostname      | PrimaryTermStartTime |
+-------------+----------------------------------+-------+------------+-------------+------------------------+---------------+----------------------+
...
| us-east1    | keyspace1 | -10   | PRIMARY    | NOT_SERVING | us-east1-0000000097    | 247.4.117.26  | 2023-10-11T19:16:00Z |
| us-east1    | keyspace1 | -10   | REPLICA    | NOT_SERVING | us-east1-0000000102    | 247.2.54.58   |                      |
...
+-------------+----------------------------------+-------+------------+-------------+------------------------+---------------+----------------------+
68 rows in set (0.00 sec)

With this change we should at least return an error message of primary is not serving, there could be a reparent operation in progress which is more informational than a "potential" resharding message.

[vitess-bot]

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