BUG: Fixes #2547 a undefined error for $originalDisplayErrorsValue

The changes made in commit dd49834 to install.php5 make use of the variable $originalDisplayErrorsValue in the function suggestPHPSetting
However as this variable will be out of scope in the function the following undefined error is being displayed on the install page

Notice: Undefined variable: originalDisplayErrorsValue in /opt/local/apache2/htdocs/campaignmonitor/framework/dev/install/install.php5 on line 511

This pull request which will store this value in a session could use a global variable but I would prefer not too even for a install script which is only used once then deleted, but I am open to suggestions and comments if anyone has a better suggestion.