id,node_id,number,state,locked,title,user,body,created_at,updated_at,closed_at,merged_at,merge_commit_sha,assignee,milestone,draft,head,base,author_association,auto_merge,repo,url,merged_by
509469020,MDExOlB1bGxSZXF1ZXN0NTA5NDY5MDIw,4533,closed,0,GH4228 Clearer error on scalar to dataframe,15890747,"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).",2020-10-24T17:52:09Z,2020-10-26T08:51:39Z,2020-10-26T08:42:35Z,2020-10-26T08:42:35Z,79df665ae77b0e01822bdf158eb27b91b8ac0591,,,0,ca07e2dc746ef1f35149eba91cb0147607b82305,1597e3a91eaf96626725987d23bbda2a80d2bae7,CONTRIBUTOR,,13221727,https://github.com/pydata/xarray/pull/4533,