Throw exceptions when index names are too long

This was fixed upstream for 3.0.0 but we're still languising on 2.3.X.
See https://rails.lighthouseapp.com/projects/8994/tickets/5645-rails-300-add_index-fails-silently-on-invalid-index-names
Merge pull request #19 from square/missing-indices-are-errors

Missing indices are errors