sql: Valid IDs in INSPECT SHARD (#31180)

Previously, INSPECT SHARD would panic if it was given an ID that didn't
belong to any item. This commit fixes the issue by returning an error
instead.

Fixes #MaterializeInc/database-issues/issues/8910

Author: jkosh44

src/sql/src/plan/statement/scl.rs

@@ -151,7 +151,8 @@ pub fn plan_inspect_shard(
     // Always inspect the shard at the latest GlobalId.
     let gid = scx
         .catalog
-        .get_item(&id)
+        .try_get_item(&id)
+        .ok_or_else(|| sql_err!("item doesn't exist"))?
         .at_version(RelationVersionSelector::Latest)
         .global_id();
     Ok(Plan::InspectShard(InspectShardPlan { id: gid }))

test/sqllogictest/shard_errors.slt

@@ -41,3 +41,6 @@ INSERT INTO bar VALUES (1);
 # Make sure we get the error even if we project away all columns.
 query error division by zero
 SELECT count(*) FROM baz;
+
+query error item doesn't exist
+INSPECT SHARD 'u666'