gir-rustdoc: Improve error message when running from working copy
When running gir-rustdoc locally without any arguments one gets
$ ./gir-rustdoc.py html-index
Traceback (most recent call last):
File "./gir-rustdoc.py", line 447, in <module>
main()
File "./gir-rustdoc.py", line 444, in main
args.func(args)
File "./gir-rustdoc.py", line 299, in html_index
project_url=args.project_url.rstrip('/'),
^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'rstrip'
Improve the error message in that case