KeyError for excessive plural forms

The latest Hausa translation for nautilus in Jaunty did something funny: it tried to translate a message to 28 plural forms! Launchpad only supports 6 forms, so at form 6 (counting from 0) we get a KeyError in the gettext parser instead of a proper error report to the user:

https://pastebin.canonical.com/12894/

In POParser._dumpCurrentSection, one of the arguments to self._message.addTranslation is self._plural_form_mapping[self._plural_case], which in this case doesn't exist.

Various syntax-colouring stop recognizing this as valid after plural form 9. If that is correct, then forms 6—9 inclusive should be reported as unsupported in Launchpad and forms 10 or higher perhaps as syntax errors.

It'd be best to intercept this weirdness right where it lands, where _parseFreshLine recognizes that msgstr is followed by an opening bracket ("if line.startswith('['):") and before it checks that the plural numbers are nearly consecutive.

The offending message is at https://pastebin.canonical.com/12893/

Original upload is at http://launchpadlibrarian.net/21460184/ha.po