Travis: Initiating build for commit '52f47a9fc38016e99097af81dad6012e4451903e' on branch 'master': 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.