python: fix error message in JobspecV1.per_resource

Problem: If neither ncores or nnodes is specified in the JobspecV1
per_resource constructor, an unhelpful TypeError is raised instead
of a useful error.

Catch the case where neither ncores or nnodes is specified to this
function and raise a specific error message.

Fixes #4547

Author: grondo

src/bindings/python/flux/job/Jobspec.py

@@ -746,6 +746,8 @@ def per_resource(
             #  With nnodes, nslots is slots/node (total_slots=slots*nodes)
             nslots = 1
             slot_size = 1
+        else:
+            raise ValueError("must specify node or core count with per_resource")
 
         children = [cls._create_resource("core", slot_size)]
         if gpus_per_node: