Improve parser error messages

locale/translations.go

locale/translations/de_DE.json

locale/translations/fr_FR.json

locale/translations/pl_PL.json

locale/translations/zh_CN.json

reader/atom/parser.go

reader/feed/handler.go

reader/json/parser.go

reader/opml/parser.go

reader/rdf/parser.go

reader/rss/parser.go