BUG: Handle download/install error for extension dependencies.

The function downloadAndInstallExtension() did not handled the case of
downloadExtension() returning 0. This commit implements the same approach
as in scheduleExtensionForUpdate() to handle that case correctly in
downloadAndInstallExtension().

To avoid overwhelming the user with a lot of different messages when
some of all dependent extensions failed to download, error messages are
accumulated in a list and shown only outside the loop if it applies.

Reviewed-by: Andras Lasso <lasso@queensu.ca>
Reviewed-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Francois Budin <fbudin@unc.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24666 3bd1e089-480b-0410-8dfb-8563597acbee