Catch the potential errors when "#__postinstall_messages" is missing (case of Upgrade)

When we proceed with a manual upgrade from Joomla 2.5.17 to J3.2.1, the #__postinstall_messages is not present and cause the impossiblity to login due to the "cPanel" that always abort on DB Error.

The purpose of this fix is to intercept such kind of error and let the cPanel displayed without any "Post Install Messages".