`raise self.error(..)` is not how to handle errors in a Docutils Domain

```
Traceback (most recent call last):lengapi/badpenny                                                                                                                                                                                                                       
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/bin/relengapi", line 9, in <module>
    load_entry_point('relengapi==0.3', 'console_scripts', 'relengapi')()
  File "/home/dustin/code/relengapi/t/relengapi/relengapi/lib/subcommands.py", line 62, in main
    args._subcommand.run(parser, args)
  File "/home/dustin/code/relengapi/t/relengapi/relengapi/blueprints/docs/__init__.py", line 134, in run
    support.build()
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/websupport/__init__.py", line 127, in build
    app.build()
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/application.py", line 220, in build
    self.builder.build_update()
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 214, in build_update
    'out of date' % len(to_build))
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 276, in build
    self.write(docnames, list(updated_docnames), method)
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 320, in write
    self._write_serial(sorted(docnames), warnings)
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 331, in _write_serial
    doctree = self.env.get_and_resolve_doctree(docname, self)
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/environment.py", line 1117, in get_and_resolve_doctree
    self.resolve_references(doctree, docname, builder)
  File "/home/dustin/code/relengapi/t/relengapi/sandbox/lib/python2.7/site-packages/sphinx/environment.py", line 1348, in resolve_references
    typ, target, node, contnode)
  File "/home/dustin/code/relengapi/t/relengapi/relengapi/lib/apidoc.py", line 331, in resolve_xref
    raise self.error("MISSING REFERENCE: api:%s:%s" % (typ, target))
AttributeError: 'ApiDomain' object has no attribute 'error'
```

And no, I don't know what the right way is!