added `ValueError` for missing `grid_mapping` variable

[larsbuntemeyer]

closes #448

This implements the fix suggested by @dcherian in #448

[codecov[bot]]

Codecov Report

Merging #458 (0d44411) into main (e283a83) will increase coverage by 0.00%. The diff coverage is 87.50%.

@@           Coverage Diff           @@
##             main     #458   +/-   ##
=======================================
  Coverage   86.33%   86.34%           
=======================================
  Files          22       22           
  Lines        4180     4188    +8     
  Branches      214      214           
=======================================
+ Hits         3609     3616    +7     
- Misses        513      514    +1     
  Partials       58       58           

Flag	Coverage Δ
mypy	37.44% <0.00%> (-0.08%)	:arrow_down:
unittests	95.97% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cf_xarray/tests/test_accessor.py	94.11% <83.33%> (-0.06%)	:arrow_down:
cf_xarray/accessor.py	86.47% <100.00%> (+0.02%)	:arrow_up:

[dcherian]

Thanks @larsbuntemeyer