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
664611511,MDU6SXNzdWU2NjQ2MTE1MTE=,4261,to_zarr with append_dim behavior changed in 0.16.0 release ,41797673,closed,0,,,4,2020-07-23T16:26:16Z,2020-11-19T15:19:48Z,2020-11-19T15:19:48Z,NONE,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

Please be concise with code posted. See guidelines below on how to provide a good bug report:

- Craft Minimal Bug Reports: http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports
- Minimal Complete Verifiable Examples: https://stackoverflow.com/help/mcve

Bug reports that follow these guidelines are easier to diagnose, and so are often handled much more quickly.
-->

**What happened**:
In version 0.15.1, calling `to_zarr` on a `Dataset` with a given `append_dim` would create a new zarr if it did not already exist. In version 0.16.0, this is not the case anymore: the call fails if the zarr does not exist.

**Minimal Complete Verifiable Example**:

```python
import xarray as xr

a = xr.DataArray([1, 2], {""t"": [1, 2]}, (""t"",))
ds = xr.Dataset({""v"": a})
ds.to_zarr(""CHOOSE_PATH"", append_dim=""t"")
```

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.7 (default, May  7 2020, 21:25:33) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.18.0-177.el8.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8
libhdf5: None
libnetcdf: None

xarray: 0.16.0
pandas: 1.0.5
numpy: 1.18.1
scipy: 1.5.0
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.3.2
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.14.0
distributed: 2.14.0
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 49.2.0.post20200714
pip: 20.1.1
conda: None
pytest: 5.4.3
IPython: None
sphinx: None


</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4261/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue