Patch from Chris Shoemaker to change the collector handling of invalid

[David Hampton]

Log Message:
-----------
Patch from Chris Shoemaker to change the collector handling of invalid
exchange functions.  Add a warning about dangerous usage of
gnc:exchange-if-same.

Tags:
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gnucash-gnome2-dev

Modified Files:
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    gnucash/src/report/report-system:
        commodity-utilities.scm