psMetadata{Next,Prev}()/psMetadataLookup() shouldn't call psError()

[rhl@…]

I don't think that we want psMetadata{Next,Prev}()/psMetadataLookup() to push an error onto the
errorstack.

They are often used to iterate over all metadata items (psMetadata{Prev,Next}()) and a NULL is the
_expected_ termination of the loop.

it isn't quite so clear for psMetadataLookup(), but it's used to check if a desired keyword is present,
so the extra error message isn't really desired. We could work around this by allowing a user to set
a mark in the error stack and pop back to it, but do we need to go to these lengths? (and the error
will still have been logged)

[robert.desonia@…]

Yes, good point! Fix is in CVS trunk.

I removed the psError calls in the most nominal case for
psMetadataGet{Next,Prev} where the metadata item end is reached. Errorneous
parameters will still produce an error.

For psMetadataLookup, I made the case where the desired keyword is not present
to not call psError. Again, errors in parameters (e.g., a NULL metadata or key
parameter) still produce an error.

-rdd