BUG: Add mixing dimension name to error message (#1241)
BUG: Add mixing dimension name to error message (#1241)

Bugfix: error message for `KeyError` has spurious `%r`, into which the dimension name is apparently intended to be substituted, but wasn't.  Substitute the dimension name so that the error message is more informative.
rabernat/xray,pydata/xarray,chunweiyuan/xarray,jhamman/xray,xray/xray,shoyer/xray,markelg/xray,shoyer/xarray,shoyer/xarray,markelg/xray,jhamman/xarray,markelg/xray,rabernat/xray,jhamman/xarray,jcmgray/xarray,pydata/xarray,pydata/xarray,jhamman/xarray