Adding show_session_error support to inform the user of any write errors

[Neil Williams]

Log Message:
-----------
Adding show_session_error support to inform the user of any write errors 
reported by any backend.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/app-file:
        gnc-file.c
        gnc-file.h
    gnucash/src/backend/file:
        gnc-book-xml-v2.c
        io-gncxml-v2.c
    gnucash/src/backend/qsf:
        qsf-backend.c
    gnucash/src/business/business-gnome:
        gnc-plugin-business.c
    gnucash/src/engine:
        qof_book_merge.c
        qofbackend.h
    gnucash/src/gnome:
        Makefile.am
        dialog-chart-export.c
    gnucash/src/gnome/glade:
        chart-export.glade