Make BulkIndexError and ScanError serializable

[seagrine]

I encountered an issue with pickling/unpickling BulkindexError.

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

I'm adding an implementation of __reduce__ to this (and ScanError) so they can be pickled and unpickled correctly. Added tests for it as well.