ResourceLoader: Stop passing around errors as /**/-comments

ResourceLoader: Stop passing around errors as /**/-comments

They are prone to being stripped by CSS and JS minification and can't
be used for non-CSS non-JS responses, like images.

We've already been keeping some state related to errors in the
$hasErrors member variable, let's go all the way and replace it with
$errors array, which tracks all errors accumulated during current
respond() call. When processing completes, all errors are added to the
response, if possible.

Change-Id: I6643f010174cb1f17780622e8a63db03cffe19e1