Fix for #1093360.

Fix for #1093360.
The fix also includes the move from manual generated error messages in
imap_general to the central error handler.
In order to catch non fatal error message in compose.php I had to collect
the error messages array before the header redirect and write them to the
session. In read_body.php, compose.php and right_main.php I add the error
messages to the error handler again and clean up the delayed error messages
array.
I also modified the error handler in order to display the page_header in
case it wasn't displayed yet.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@10705 7612ce4b-ef26-0410-bec9-ea0150e637f0