Python doc generator: raise explanatory error when context key is missing

When a new key is added to the context, it also needs to be added to the
`sphinx_doc_gen.py` file for generating the Python API documentation.
When this isn't done, the script would raise a generic `KeyError`. Now
it explains what needs to be updated to solve the problem.

No functional changes to Blender.

doc/python_api/sphinx_doc_gen.py

@@ -1223,7 +1223,10 @@ def pycontext2sphinx(basepath):
         while char_array[i] is not None:
             member = ctypes.string_at(char_array[i]).decode(encoding="ascii")
             fw(".. data:: %s\n\n" % member)
-            member_type, is_seq = context_type_map[member]
+            try:
+                member_type, is_seq = context_type_map[member]
+            except KeyError:
+                raise SystemExit("Error: context key %r not found in context_type_map; update %s" % (member, __file__)) from None
             fw("   :type: %s :class:`bpy.types.%s`\n\n" % ("sequence of " if is_seq else "", member_type))
             unique.add(member)
             i += 1