Warn on failed templates_c write

Overview

I think this is a problem that we've all faced at some point or another, but experienced implementers know how to fix this, and newbies get caught. When some of your templates_c files are owned by a user that's not the user running PHP, Civi silently fails in weird and wonderful ways.

Reproduction steps

1. Recursively chown your templates_c folder such that the PHP user doesn't have write access.

Current behaviour

Parts of the Smarty forms fail to render.

Expected behaviour

Some sort of warning that there's a permissions issue on the templates_c folder.

I don't mind doing this if it gets "Concept Approved", though admittedly I've never dug that far down. I'm worried that Smarty isn't going to bubble up an exception, but it's worth a look.