Feature: Information about forum state

**Community discussion about this feature (mandatory)**
CA & dev

**Is your feature request related to a problem? Please describe.**
Forum down

**Describe the solution you'd like**
If the forum returns a 500 server error, instead of "Network error", show a friendlier message to users (message on the maintenance page of the forum is ok). Should be a transifex string to be translated in the differents languages.
This for the forum widget, board announcement and site notice

Start by inspecting the forum widget, board announcement, and site notice entry points, especially where a forum 500 response becomes "Network error." Check how existing Transifex strings are defined and used. Done means all three surfaces show a friendlier translatable message when the forum is unavailable.

Written by the indexing model from the issue text.