Replace all uses of Piwik_ExitWithMessage (except one) with throwing exceptions & include HtmlMessageException class for exceptions w/ HTML content in their message.
Remove ExitWithMessage function and documented new code.
Adding event so the error page contents can be modified.
Use correct exception for auth error in FrontController.
Fix typo.
Merge pull request #6541 from piwik/remove_exitwithmessage

Use exceptions instead of calls to Piwik_ExitWithMessage. Also includes refactoring Piwik_ExitWithMessage so no echo-ing or exit-ing is done by the function (function is renamed), modification to Log.php so it will work even if config is empty, and, includes event that allows modification of error page contents.