[Chore](load) return error reason when database/table not founded whe…

…n stream load (#38967)

## Proposed changes
return error reason when database/table not founded when stream load

fe/fe-core/src/main/java/org/apache/doris/httpv2/rest/LoadAction.java

@@ -62,6 +62,7 @@
 import java.net.URI;
 import java.util.Enumeration;
 import java.util.List;
+import java.util.Optional;
 import java.util.Set;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
@@ -293,8 +294,17 @@ private Object executeWithoutPassword(HttpServletRequest request,
                     return new RestBaseResult(e.getMessage());
                 }
             } else {
-                long tableId = ((OlapTable) ((Database) Env.getCurrentEnv().getCurrentCatalog().getDb(dbName)
-                        .get()).getTable(tableName).get()).getId();
+                Optional<?> database = Env.getCurrentEnv().getCurrentCatalog().getDb(dbName);
+                if (!database.isPresent()) {
+                    return new RestBaseResult("Database not founded.");
+                }
+
+                Optional<?> olapTable = ((Database) database.get()).getTable(tableName);
+                if (!olapTable.isPresent()) {
+                    return new RestBaseResult("OlapTable not founded.");
+                }
+
+                long tableId = ((OlapTable) olapTable.get()).getId();
                 redirectAddr = selectRedirectBackend(request, groupCommit, tableId);
             }