added error trapping to nc_to_df to check space-time representation

[dklinges9]

Hi @everydayduffy,

Resubmitting an updated pull request after our convo on my last pull request (now closed). A few changes on this pull request:

• as you requested, I've added this to R/internal.R
• the error trapping is in both nc_to_df and nc_to_df_precip
• note that there's two commits on this request: the first commit is squashing a bug I noticed from when you changed to arguments from x/y to lat/long, there were a few spots that weren't updated to this yet

Hope this helps!

[everydayduffy]

Thanks for this, all merged.