Semi-determinism in Exempi

[Pipian]

In the process of fiddling a bit more with Exempi, I've worked around  
a minor bug that I'm still trying to fix (I'll send a patch when I  
can), and discovered a rather unpleasant side-effect of Exempi's  
exception handling of the internal C++ code:  Many functions that  
return boolean values will return false if the main function returns  
false AND when an exception is caught.  This in and of itself would  
not be a problem, but there is no way to determine when a genuine  
error has occurred and when the function is returning false, as the  
error code is not reset between function calls.

For example, if there is an error in obtaining a property value with  
xmp_get_property(), it is indistinguishable from when no property is  
found, as there is no way to know whether or not xmp_get_error() was  
set during xmp_get_property().  This makes it difficult to write unit  
tests when expecting no property to be found, as there is no way to  
ensure that an error did not occur when reading the property.

I thus propose that exempi add a new error code XMPErr_OK to which the  
error variable is set at the beginning of each function.  This way, it  
is possible to determine if a genuine error has occurred or whether  
the function is merely returning false.

Ian