Web: wrong use of error_page() function throughout the code

[ChristianBeer]

Describe the bug
The current implementation of error_page() in html/inc/util.inc#L367 contains a call to page_head() which among other things outputs header information (via header(). This means that after a page called page_head() itself it can't use the current error_page() function or the header is output twice. This leads to duplication of HTML code (two navbars) and PHP warnings Cannot modify header information - headers already sent by.

Steps To Reproduce

1. Create a page that calls error_page() after page_head()
2. View this page with a browser

Expected behavior
There should be no warnings and only one vaigation bar on the top. Therefore the second call to page_head() when calling error_page() after page_head() should be avoided. When error_page() is called before page_head() then of course it needs to call page_head() itself.

Scope
This is a rather big change as one has to check each PHP file and modify the error_page() call that happens after a page_head() call. So I would propose to do this in chunks and document progress in this issue.