Upgrade code reference warnings to errors.

Expected Behavior

I don't know if it's "expected" behavior per say, of feature/change request.

I'd like a way to have ReferenceWarning: Code reference mymodule.erroneous_reference in module "mymodule" does not match any documented object. be an error, so that my CI action catches this sort of problem.

Currently, it seems only import errors are considered "errors", as per --skip-error. I would expect that erroneous references should be errors as well, that can be downgraded to warning using --skip-error.

I can do a quick PR if that's ok with you :)

Actual Behavior

def test() -> "mymodule.erroneous_reference":
  pass

or

def test():
    """Hello world

    Returns:
        mymodule.erroneous_reference
    """
    pass

gives ReferenceWarning: Code reference mymodule.erroneous_reference in module "mymodule" does not match any documented object.

which is a warning, regardless if skip-errors is set.

Steps to Reproduce

def test() -> "mymodule.erroneous_reference":
  pass

Additional info

• pdoc version: latest, master

Search for ReferenceWarning and the --skip-error handling, then reproduce the issue with the annotated return type or docstring examples. Done means erroneous code references are treated as errors by default and can be downgraded to warnings with --skip-error.