Protect against NSXMLParserTagNameMismatchError
Protect against NSXMLParserTagNameMismatchError

When adding a new feed with NSXMLDocumentTidyXML, some types of errors cause
an unhandled exception. I added NSXMLParserTagNameMismatchError as another case
which we should fail on when parsing a feed.

Closes #1073

barijaona/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,josh64x2/vienna-rss,Eitot/vienna-rss,ViennaRSS/vienna-rss,lapcat/vienna-rss,Eitot/vienna-rss,josh64x2/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,lapcat/vienna-rss,barijaona/vienna-rss,josh64x2/vienna-rss,lapcat/vienna-rss,barijaona/vienna-rss,Eitot/vienna-rss,ViennaRSS/vienna-rss,lapcat/vienna-rss,Eitot/vienna-rss,barijaona/vienna-rss