better error messages for data types pretending to be data.frame but really aren't

[markvanderloo]

See #17

[njtierney]

Hi @markvanderloo,

I appreciate the problems in #17 - I was wondering if you would be interested in a PR that helps simputation work with the new tibble ? I understand if you prefer to cast to data.frame instead, but just wanted to see if I could maybe help out.

[markvanderloo]

Hey Nick,

Sure a PR would be fine!

Mark