Internationalize unexpected error message (#6887)

...astodon/features/ui/containers/notifications_container.js

app/javascript/mastodon/locales/ar.json

app/javascript/mastodon/locales/bg.json

app/javascript/mastodon/locales/ca.json

app/javascript/mastodon/locales/de.json

app/javascript/mastodon/locales/defaultMessages.json

app/javascript/mastodon/locales/en.json

app/javascript/mastodon/locales/eo.json

app/javascript/mastodon/locales/es.json

app/javascript/mastodon/locales/fa.json

app/javascript/mastodon/locales/fi.json

app/javascript/mastodon/locales/fr.json

app/javascript/mastodon/locales/gl.json

app/javascript/mastodon/locales/he.json

app/javascript/mastodon/locales/hr.json

app/javascript/mastodon/locales/hu.json

app/javascript/mastodon/locales/hy.json

app/javascript/mastodon/locales/id.json

app/javascript/mastodon/locales/io.json

app/javascript/mastodon/locales/it.json