Fix manifest error handling

This corrects an extra argument in the error message and performs a
check to make sure the asynchronous operation is running. Some cases,
such as malformed XML file, can return immediately, leaving the
override cursor set and selection dialog disabled.

Change-Id: I6b9b95a9acc3a79bed6cca22d859832567819e44
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>