Use precondition failed code in delete_timeline when tenant is missing (#3884)

This allows client to differentiate between missing tenant and missing
timeline cases

pageserver/src/http/openapi_spec.yml

@@ -189,6 +189,13 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/NotFoundError"
+        "412":
+          description: Tenant is missing
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/PreconditionFailedError"
+
         "500":
           description: Generic operation error
           content:
@@ -958,6 +965,13 @@ components:
       properties:
         msg:
           type: string
+    PreconditionFailedError:
+      type: object
+      required:
+        - msg
+      properties:
+        msg:
+          type: string
 
 security:
   - JWT: []

pageserver/src/http/routes.rs

@@ -148,6 +148,11 @@ impl From<crate::tenant::mgr::DeleteTimelineError> for ApiError {
     fn from(value: crate::tenant::mgr::DeleteTimelineError) -> Self {
         use crate::tenant::mgr::DeleteTimelineError::*;
         match value {
+            // Report Precondition failed so client can distinguish between
+            // "tenant is missing" case from "timeline is missing"
+            Tenant(TenantStateError::NotFound(..)) => {
+                ApiError::PreconditionFailed("Requested tenant is missing")
+            }
             Tenant(t) => ApiError::from(t),
             Timeline(t) => ApiError::from(t),
         }