User callbacks to `with_transaction` are expected to return false and set a `bson_error_t` in case of failure.  However, if a user callback is reporting a user-side error in their own code (not reporting back a different libmongoc error), there are no error codes/domains they can use to represent their failure.  Let's either provide some way to signal an "unknown error" to the driver, or document that users should set the error code and domain to 0 or another well known value.