US1: Output error messages through $_SESSION

[BorislavZlatanov]

Related user story:
As an administrator, I can add new resources to the system.

Task:
On the page for adding resources, right now we are outputting error messages through 'echo'. We instead need to save them to $_SESSION, redirect from the form processor file back to the form and output error messages there (if any).

[BorislavZlatanov]

I was thinking recently about this and whether it would be better to submit the form through AJAX. That way all the inserted values in the form will be preserved (the user wouldn't have to type them again if there are errors) and we can just insert at the top of the page whatever error messages get returned by the server.