... procedure call.
This is needed for plug-ins which depends on other plug-in's procedures.
If for instance, the second-level plug-in is interrupted interactively,
we don't want to process this as an error but as a cancellation.
Therefore we need to know the returned value of the plug-in. Currently
only way was to use gimp_get_pdb_error() but that was returning a
human-readable error, not a computer-processable error.