id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
728840803,MDExOlB1bGxSZXF1ZXN0NTA5NDY5MDIw,4533,GH4228 Clearer error on scalar to dataframe,15890747,closed,0,,,4,2020-10-24T17:52:09Z,2020-10-26T08:51:39Z,2020-10-26T08:42:35Z,CONTRIBUTOR,,0,pydata/xarray/pulls/4533,"When attempting to convert a scalar to a dataframe, as e.g.:

`xarray.DataArray([1], coords=[('onecoord', [2])]).sel(onecoord=2).to_dataframe(name='name')`

raise 

`ValueError: cannot convert a scalar to a Dataframe`

instead of 

`ValueError: no valid index for a 0-dimensional object`


 - [x] Closes #4228
 - [x] Tests added
 - [x] Passes `isort . && black . && mypy . && flake8`.  
 - [x] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
  > I was not sure if this was ""notable"". Please let me know if I should add.
 - [ ] New functions/methods are listed in `api.rst`
> n / a

I was not able to run all tests, but was able to run the changed one (more on that in the issue).","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4533/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull