map_over_datasets: fix error message for wrong result type

[mathause]

The error message did not indicate the name of the node which is fixed here.

It shows '.' for the root node while the repr uses / which is maybe confusing. This comes from using NodePath() - maybe this should be changed to NodePath("/")?

xarray/xarray/core/treenode.py

Line 432 in c252152

queue = collections.deque([(NodePath(), self)])

[mathause]

Ok, I also found it helpful to have the result on a new line.

before:

TypeError: the result of calling func on the node at position 'ssp126' is not a Dataset or None or a tuple of such types: <xarray.DataArray 'selected_ar_order' (member: 1)> Size: 8B
array([2.])
Coordinates:
  * member   (member) <U8 32B 'r1i1p1f1'

now:

TypeError: the result of calling func on the node at position 'ssp126' is not a Dataset or None or a tuple of such types:
<xarray.DataArray 'selected_ar_order' (member: 1)> Size: 8B
array([2.])
Coordinates:
  * member   (member) <U8 32B 'r1i1p1f1'