Messages are generated too early

Shortened that comment.

Problem

• Status/error messages triggered in MODULE_preprocess_page() and THEME_preprocess_page() are not displayed.

Goal

• Generate and render status/error messages as late as possible.

Details

• The $messages variable is generated relatively early in template_preprocess_page() already.
• The generated/rendered variable only contains messages that have been set so far.
• Any messages that are triggered/set in later preprocess hooks are not output (resp. output on the next page request instead of the current).
• At least for themes, it may be relatively common to use THEME_preprocess_page() to output a message on certain pages.

Notes

• This patch only accounts for messages being output in later preprocess hooks. Messages triggered in process hooks or during render($variable) within the template file itself are still not captured.
• I don't think it makes sense to test this change, as we are still doing the identical thing, just slightly later in time. We need to find a proper solution for this in D8.

API impact

• Currently, it is technically possible to alter messages in hook_preprocess_page(). Modules or themes that are implementing this hook in order to alter status/error messages need to implement hook_process_page() instead.

Some further notes:

- The outlined API change impact, although being true, can be considered moot, because $messages is a HTML string already, so all that a hook_preprocess_page() is able to do with regard to $messages is performing crazy-strange string replacements.

- Messages also means error messages. That's the primary reason for why I think this patch is important.