sequence: replace assertion with diag in sequence_data_iterator_create

We aren't planning to support all iterator types for _sequence read
views, but once we implement a Lua API for read views, the user will
still be able to create a read view of this space. So let's replace
the assertions with a graceful error.

Needed for https://github.com/tarantool/tarantool-ee/issues/197

NO_DOC=internal
NO_TEST=ee
NO_CHANGELOG=internal