Don't block a command while retrieving parts if we can afford it.

If the relevant parts are not available, and the ItemFetchJob will
simply block until ItemRetriever retrieved each individual message
from the resource. This completely blocks i.e. the kmail etm session.
By simply avoiding this if we can afford it (ignoreErrors, which the
ETM sets), we can circumvent this problem.

We may still want to trigger a non-blocking process that retrieves the
missing parts though.