BUG: Fixed extension install failure

Extension installation silently failed after a previous extension installation was partially completed due to running out of disk space.

1. No errors were displayed => Added error display for all failure branches in qSlicerExtensionsManagerModel::extractExtensionArchive

2. SlicerIGT-XXXXXX intermediate directory was left in the extension directory, that prevented all further attempts to move the extracted archive content to the correct location (directory copy fails if the target directory is not empty) => Added deletion of the intermediate directory before copying.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24694 3bd1e089-480b-0410-8dfb-8563597acbee