fixes #1457 - remove E_EXCEPTION (couldn't find any documentation for this as a predefined PHP constant)

refs #998 - merge upgrade.php and common.php so json_decode() is available to Tracker plugins; sync up with -17 release of upgradephp


git-svn-id: http://dev.piwik.org/svn/trunk@2420 59fd770c-687e-43c8-a1e3-f5a4ff64c105