fixes #1320 - output tracking pixel at start of request to increase perceived tracker responsiveness

the tradeoff is the loss of some user-readable text in non-debug mode for empty requests and exceptions

(arguably, we probably shouldn't have been outputing e->getMessage() in non-debug mode)



git-svn-id: http://dev.piwik.org/svn/trunk@6539 59fd770c-687e-43c8-a1e3-f5a4ff64c105