BulkIndexError cannot be unpickled

[seagrine]

It seems like BulkIndexError cannot be unpickled:

unpickle_exception\n inst = func(*args)\n ^^^^^^^^^^^\nTypeError: BulkIndexError.**init**() missing 1 required positional argument: 'errors

This creates problems with some libraries that expect errors to be serializable.

I put together a quick PR to fix this.

[seagrine]

#2669