stubtest: use single line error message (#13301)

This preserves the invariant that in `--concise` the problematic objects
are the start of each line. It also matches the pattern we have for e.g.
the "explicitly passing in defaults" error message. Also add some comments.

mypy/stubtest.py

@@ -246,6 +246,8 @@ def verify(
 def _verify_exported_names(
  object_path: List[str], stub: nodes.MypyFile, runtime_all_as_set: Set[str]
 ) -> Iterator[Error]:
+ # note that this includes the case the stub simply defines `__all__: list[str]`
+ assert "__all__" in stub.names
  public_names_in_stub = {m for m, o in stub.names.items() if o.module_public}
  names_in_stub_not_runtime = sorted(public_names_in_stub - runtime_all_as_set)
  names_in_runtime_not_stub = sorted(runtime_all_as_set - public_names_in_stub)
@@ -254,16 +256,13 @@ def _verify_exported_names(
  yield Error(
  object_path,
  (
- "module: names exported from the stub "
- "do not correspond to the names exported at runtime.\n"
- "(Note: This is probably either due to an inaccurate "
- "`__all__` in the stub, "
- "or due to a name being declared in `__all__` "
- "but not actually defined in the stub.)"
+ "names exported from the stub do not correspond to the names exported at runtime. "
+ "This is probably due to an inaccurate `__all__` in the stub or things being missing from the stub."
  ),
- # pass in MISSING instead of the stub and runtime objects,
- # as the line numbers aren't very relevant here,
- # and it makes for a prettier error message.
+ # Pass in MISSING instead of the stub and runtime objects, as the line numbers aren't very
+ # relevant here, and it makes for a prettier error message
+ # This means this error will be ignored when using `--ignore-missing-stub`, which is
+ # desirable in at least the `names_in_runtime_not_stub` case
  stub_object=MISSING,
  runtime_object=MISSING,
  stub_desc=(