storcon: don't error log on timeline delete if tenant migration is in progress (#12523)

Fixes [LKB-61](https://databricks.atlassian.net/browse/LKB-61):
`test_timeline_archival_chaos` being flaky with storcon error `Requested
tenant is missing`.

When a tenant migration is ongoing, and the attach request has been sent
to the new location, but the attach hasn't finished yet, it is possible
for the pageserver to return a 412 precondition failed HTTP error on
timeline deletion, because it is being sent to the new location already.
That one we would previously log via sth like:

```
ERROR request{method=DELETE path=/v1/tenant/1f544a11c90d1afd7af9b26e48985a4e/timeline/32818fb3ebf07cb7f06805429d7dee38 request_id=c493c04b-7f33-46d2-8a65-aac8a5516055}: Error processing HTTP request: InternalServerError(Error deleting timeline 32
818fb3ebf07cb7f06805429d7dee38 on 1f544a11c90d1afd7af9b26e48985a4e on node 2 (localhost): pageserver API: Precondition failed: Requested tenant is missing
```

This patch changes that and makes us return a more reasonable resource
unavailable error. Not sure how scalable this is with tenants with a
large number of shards, but that's a different discussion (we'd probably
need a limited amount of per-storcon retries).

example
[link](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-12398/15981821532/index.html#/testresult/e7785dfb1238d92f).

storage_controller/src/service.rs

@@ -5206,6 +5206,9 @@ impl Service {
                 match res {
                     Ok(ok) => Ok(ok),
                     Err(mgmt_api::Error::ApiError(StatusCode::CONFLICT, _)) => Ok(StatusCode::CONFLICT),
+                    Err(mgmt_api::Error::ApiError(StatusCode::PRECONDITION_FAILED, msg)) if msg.contains("Requested tenant is missing") => {
+                        Err(ApiError::ResourceUnavailable("Tenant migration in progress".into()))
+                    },
                     Err(mgmt_api::Error::ApiError(StatusCode::SERVICE_UNAVAILABLE, msg)) => Err(ApiError::ResourceUnavailable(msg.into())),
                     Err(e) => {
                         Err(