1. During the exporting, put the error messages into $SESSION.
2. Refactor the displayMessages() function to display all messages
or hide the previous one.
3. Refactor the function print_export_footer() to handle html code in
lang strings.

Change-Id: I589822dc360e8f5bcac0298f9534824513214160
Signed-off-by: Son Nguyen <son.nguyen@catalyst.net.nz>