i18n: consistency tweaks

- Add message "wikiforum-error-general-title" which is used as error title.
- Add full-stops to error messages.
- Split into two sentences.

Spotted by Siebrand
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikiforum-cat-not-found-text/en
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikiforum-error-readonly/en
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikiforum-error-write/en
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikiforum-forum-not-found-text/en
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikiforum-must-supply-thread/en
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikiforum-thread-not-found-text/en

Change-Id: I16bc4ebddd6d16204b2bd85d276a79c3497f1a9d