When Piwik config file becomes un-readable, make it clear that the file still exists but is not readable (#10331)

* * In messages, make it more clear whether config file exists and/or is (not) readable
* When the config file exists but is not readable, provide users the choice between making the file readable, or starting a fresh install
* As long as Piwik is not installed, piwik.php should not return 500 when config file is not readable
* Fixes #10283
* UI tests: updated message when config file is not found