Removed most exit() calls from the MediaWiki core, by replacing them with either...

Removed most exit() calls from the MediaWiki core, by replacing them with either a throw or by classic error checking. OutputPage::fatalError() and similar functions are deprecated, use either OutputPage::showFatalError() to return control or throw new FatalError() to relinquish it. Backwards compatibility of all interfaces has been maintained, thus extensions should continue to work and can be ported at a later date.