fix(db): throw readable error when creating _id with background: true

lib/db.js

@@ -1042,14 +1042,16 @@ var createIndex = function(self, name, fieldOrSpec, options, callback) {
     // 86 = 'IndexKeySpecsConflict' (index already exists with the same name)
     // 11000 = 'DuplicateKey' (couldn't build unique index because of dupes)
     // 11600 = 'InterruptedAtShutdown' (interrupted at shutdown)
+    // 197 = 'InvalidIndexSpecificationOption' (`_id` with `background: true`)
     // These errors mean that the server recognized `createIndex` as a command
     // and so we don't need to fallback to an insert.
     if (
       err.code === 67 ||
       err.code === 11000 ||
       err.code === 85 ||
       err.code === 86 ||
-      err.code === 11600
+      err.code === 11600 ||
+      err.code === 197
     ) {
       return handleCallback(callback, err, result);
     }