Handle "unimplemented" roles. Fixes #1610287.

[brechtm]

The unimplemented (but defined) roles are:

• index (implemented by Sphinx)
• named-reference
• anonymous-reference
• uri-reference
• footnote-reference
• citation-reference
• substitution-reference
• target
• restructuredtext-unimplemented-role

[openstack-gerrit]

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[brechtm]

This is very confusing. So you don't accept pull requests on GitHub? Why use GitHub at all if you're not using the issue tracker and pull requests?

To be honest, I don't feel like reading the two huge pages of text linked to by @openstack-gerrit. I just wanted to contribute a simple fix. Can't it just get to the point and tell me what's going on?