follow up r62231, r61779, r62175

follow up r62231, r61779, r62175
* Fix up messages
* For new FileRepo::append(), use flags to determine whether to delete or not
* Add more error checking for appending
* Fix a couple of places in Revision.php and LogPage.php where DB errors were produced when comment was null
* Remove bogus checking for !$comment, etc on the DONE phase of chunked uploading
* Don't pretend to return a value when raising an exception
* Add more tests for chunked uploads
* Verify that Status::getErrorsArray() (at least where it is used in ApiUpload::execute()) returns an array that we can pass to dieUsageMessage()
* Ensure that checkWarnings(), etc work only on the complete file