Make WikiPage::doDeleteArticle more robust

Make WikiPage::doDeleteArticle more robust

When it becomes impossible to load the content of a page due to some
error or misconfiguration, we still want to be able to delete that
page. This change makes WikiPage::doDeleteArticle more robust by catching
any exceptions that may be thrown while trying to load the page content
during the deletion process.

See T128466 for context.

Change-Id: I19f2d16850a3c1af5b504a70a27b9bf1330bc68d