Print a better error when an invalid timestamp is in a data file.

[David Hampton]

Log Message:
-----------
Print a better error when an invalid timestamp is in a data file.

Tags:
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gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/backend/file:
        gnc-pricedb-xml-v2.c
        gnc-transaction-xml-v2.c
        io-gncxml-v1.c
        sixtp-dom-parsers.c
        sixtp-dom-parsers.h
    gnucash/src/backend/file/test:
        test-dom-converters1.c
    gnucash/src/business/business-core/file:
        gnc-entry-xml-v2.c
        gnc-invoice-xml-v2.c
        gnc-order-xml-v2.c