mongo_create_index is expected to return a bson object in case of errors

The behavior is described in the header comment and matches the contract of mongo_cmd_get_last_error. Callers should use bson_has_data(.) to check.

This avoids a segfault in my code which was caused by these new checks in 1abad57.
Merge pull request #146 from paulmelnikow/mongo_create_index_fix

mongo_create_index is expected to return a bson object in case of errors