Removed @ operator from dol_include_once

Adding @ operator to include_once makes include errors untraceable, it shouldn't be there. Errors must be thrown so that we notice them. It is PHP configuration's choice to decide wether to log them or print them.
Avoiding include in case of unexisting file
Corrected file_exists check and little refactor
Merge pull request #1746 from marcosgdf/patch-1

Removed @ operator from dol_include_once