Actually check sourceFile for failure, showing the error message in the install.

Actually check sourceFile for failure, showing the error message in the install.
See report on http://permalink.gmane.org/gmane.org.wikimedia.mediawiki/33902
Fixed several tag nesting problems from setup_database().
Make sourceFile() always use text errors in the installer. It can't handle
exceptions properly (introduced in r36211). But they are appropiate for update.php
thus the ugly branches.
The installer now always handles sourceFile() errors.