Exception: Clean up html document for error pages

Interface:
* 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/>.

Clean up:
* 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.

Coding style:
* 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.

Change-Id: I33232d871200cbd23501c9a6c5f8a178931e135e
Merge "Exception: Clean up html document for error pages"