Exception: Clean up html document for error pages
- Restore sitename as part of error page document title (follows-up 4ca9805). Moved to general Exception class as it is useful in general, this effectively changes the title from "This wiki has a problem" to "Internal error - Wikipedia".
- Added basic <style> to have it use a sans-serif font and a bit padding to offset it from the edge of the window.
- Output stacktrace in <pre> as-is (with linebreaks) instead of in a <p> with <br/>.
- Removed spurious "<!-- SiteSearch Google -->" comment.
- Change sitesearch radio button to not need the id/for attributes but simply nest them. Visual rendering and browser behaviour is identical.
- Renamed $text to $html since that is what it is (follows-up 4ca9805 which reused that variable for html, it used to contain text).
- Remove odd linebreak from "dberr-problems" message, it was being output as-is in the html (html escaped, of course) and line breaks have no meaning inside an <h1> tag. Using a simple space instead. Visual rendering is identical.
- Using <!DOCTYPE html> instead of <!doctype html> for consistency with other documents we output.
- Switched a few inline #-style comments to use // instead.
- Switched a few strings from double quotes to single quotes in areas close to the changed code.